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Nematodes use stylets to break the plant cell wall.

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Nematodes use stylets to break the plant cell wall.

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Plant Parasitic nematodes | Stylet | colonization of roots

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World of Plants Parasitic Nematodes | Forms and Kinds | Symptoms on Plants

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In this edition we focus on plant-parasitic nematodes. Professor, Driekie Fourie , Technical Product Lead Seedcare at Syngenta for Africa-Middle East chats to...

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Nematodes are thread-like roundworms that live in a wide range of environments including soil and fresh and salt water. There are species of nematodes that feed on fungi, bacteria, protozoans, other nematodes, and plants. They can also parasitize insects, humans, and animals. Nematodes that feed on plant parts are called plant parasitic nematodes (PPN) and are ubiquitous in agricultural soils. The life cycle of a nematode includes eggs, juveniles and adults, and they can overwinter at any of these stages. Crop damage is the result of a complex interaction of the environment, initial nematode populations at planting, the pathogenicity of the nematode species and the ability of the plant to tolerate nematode feeding.

Most PPNs feed by piercing and killing root cells with needle-like structures called stylets. Nematodes that utilize this type of feeding include lesion, lance, needle, sting, stunt, and sting nematodes. Some of the most economically damaging nematodes like the root knot nematode (RKN) and soybean cyst nematode (SCN) enter roots and establish a permanent feeding sites where they complete their life cycles without killing the cells around them.

Symptoms associated with nematode infection are similar to those caused by impaired root growth and function, therefore they may resemble abiotic stress like drought and nutritional deficiencies as well as biotic factors like stem and root rots. General symptoms from nematodes include yellowing, stunting, and wilting, accompanied by a yield decline. In the case of SCN, signs of infection are white-to-pale-yellow female bodies present in roots that can be seen with the naked eye (Figure 1). Nonetheless, above ground symptoms are not always obvious and infections can go undetected until populations are well-beyond economic thresholds. The RKN causes root galling, however, the degree of galling may depend on the interaction between the plant and the RKN species.

Nematode assays from soil samples are the only way to confirm the presence and population density of PPNs in your field. Once a crop is planted and roots start to grow, it will serve as the food source to support nematode reproduction and population growth. As such, the best time to monitor nematode densities in your soils is right before harvest or at harvest when nematode populations reach their highest levels. These results can be used to predict potential nematode problems in future crops. However, it should be noted that sampling can be conducted during the growing season if there is suspicion of nematodes affecting your crop. Samples collected in the spring will normally have reduced levels of nematodes due to unfavorable conditions for the pathogen and lack of host crops, and in some cases species like the RKN may not be detected.

Sampling for nematodes: Collect 1-inch-diameter soil cores to a depth of 8 inches. For sample collection follow a zigzag pattern and take soil cores from within the rows and between plants. Collect the soil cores in a bucket and mix them together to create a composite sample. Place the composite sample in a plastic bag, keep it protected from direct sunlight or heat, and store in a refrigerator until submission. Obtain at least 20 soil cores for areas of 20 acres or smaller. For patches of stunted and yellow plants, samples should be collected from the margin of the affected areas, avoiding the area where plants look heavily damaged. Additionally, you can dig out plants and send specimens to a laboratory for a more accurate diagnosis. If you intend to send plants, please contact the laboratory about sampling submission procedures. There are many public and private laboratories that conduct nematode assays.

The number of nematodes required to cause crop losses varies according to the nematode and plant species, therefore the implementation of management practices to reduce PPN impacts will based on the correct interpretations of the nematode assay results. Approaches for nematode analysis interpretation include:

1. Action thresholds: the PPN population at which management practices must be implemented to prevent economic losses.

2. Risk level: nematode populations are classified as low, moderate, or high according to the likelihood of causing crop damage.

Based on the results of the nematode assays, most laboratories will include management guidelines in their reports. However, it is important to note that thresholds and risk levels apply for samples taken at the end of the growing season and those can differ from one state to another according to specific crops, soil types, local research, and the laboratory's own experience.

Thresholds levels and recommendations from different laboratories also vary according to the amount of soil used for assays. While our results were based on 500 cm3 of soil, the available action thresholds are based on the number of nematodes in 100 or 250 cm3 of soil. The number of nematodes must be standardized per unit of volume of soil to make an adequate interpretation.

Nematode sampling in Pennsylvania

As part of a state-wide research project aimed to study soybean yield limiting factors, as well as our commitment to the national SCN Coalition, samples for nematode assays were collected in the spring of 2018 from more than 30 farms across PA. Current results indicate the presence of lesion, sting, spiral, stubby-root, lance, and dagger nematodes. Some of these PPN have been reported in soybean, corn, or both. The fact that RKN and SCN were not detected in the spring samples should not be interpreted as if they are not present in fields, since as previously stated, populations in the spring may be too low to account for all PPN and management guidelines are typically based on population levels from samples taken at harvest. We will compare the results from the spring samples (baseline data) to monitor how PPN populations changed during the growing season. Additional analyses to identify nematodes at the species level may be required to make a better interpretation of the results.

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Nematodes are thread-like roundworms that live in a wide range of environments including soil and fresh and salt water. There are species of nematodes that feed on fungi, bacteria, protozoans, other nematodes, and plants. They can also parasitize insects,

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Nematodes as Drivers of Plant Performance in Natural Systems

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Nematodes as Drivers of Plant Performance in Natural Systems

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Isolation, identification, and pathogenicity of entomopathogenic nematodes occurring in Cappadocia Region, Central Turkey

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Isolation, identification, and pathogenicity of entomopathogenic nematodes occurring in Cappadocia Region, Central Turkey

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Egyptian Journal of Biological Pest Control 29, Article number: 40 (2019) Cite this article

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Abstract

A survey for naturally occurring entomopathogenic nematodes (EPNs) was conducted in various agricultural fields in central Turkey, Nevsehir, between 2015 and 2016 years. EPNs were recovered from 20 of 112 soil samples (17.9%). Seventeen isolates were identified as Steinernema feltiae (Rhabditida: Steinernematidae). One was unknown Steinernema sp. (Rhabditida: Steinernematidae), and two were Heterorhabditis bacteriophora (Rhabditida: Heterorhabditidae). The most common species was S. feltiae, which was recovered from 5 out of 8 sites. The pathogenicity of these isolates was evaluated on the last larval instar of Galleria mellonella L. (Lepidoptera: Pyralidae) at different concentrations (25, 50, 100 IJs) in the laboratory at 25 ± 1 °C. Maximum mortality rate (87%) was achieved from the H. bacteriophora AVB-15 isolate at the lowest concentration after 48 h post-inoculation, while the only isolate that caused maximum mortality of (100%) at the concentration of 50 IJs/ml, 24 h post-inoculation was S. feltiae DDKB-17 isolate. All isolates showed high pathogenicity on G. mellonella last instar ranging from 63 to 100% at the concentration of 100 IJs/ml, 48 h post-inoculation. The present survey revealed that these EPNs are commonly present at Nevsehir, and they might have a good potential in biological control of insect pests.

Background

Long-standing improper use of pesticides has led to an increased risk of contamination of the environment and harmful effects on food security. Concerns about the destructive effects of chemical pesticides have prompted a growing number of researchers across the world to find more sustainable and environmentally friendly novel control methods that offer the production of safer foods to consumers in the management of pests (Canhilal et al. 2016). Entomopathogenic nematodes (EPNs) from the families Heterorhabditidae and Steinernematidae possess many features that distinguish them from other EPNs and enable them to provide a more successful control against insect pests that live in both soil and cryptic habitats (Hazir et al. 2004; Kaya et al. 2006 and Lacey and Georgis 2012). Recently, EPNs as biological control agents proved to be successful in the management of major insect pests in agriculture (Yuksel et al. 2018; Yuksel and Canhilal 2018 and Majić et al. 2019). Therefore, there is a great scientific interest in both obtaining EPNs and determining their efficacy against the most damaging species (Canhilal et al. 2016 and Kepenekci et al. 2018).

The occurrence and distribution of EPN have been studied in some parts of Turkey, but no study has been conducted at Nevsehir (Kepenekci and Susurluk 2000; Canhilal et al. 2016 and Canhilal et al. 2017).

The objective of the present study was to determine the distribution and species composition of EPNs at Nevsehir Province, and to evaluate their pathogenicity on Galleria mellonella L.

Materials and methods

A survey took place between 2015 and 2016 years, and soil samples were collected randomly with a hand shovel from different habitats such as forest, pasture, field crops, vegetable, and fruit orchards following rainy days in spring (March-June) and autumn (September-October) (Majić et al. 2018). A total of 112 soil samples were collected. Each one (approximately 1 kg) consisted of 6-8 subsamples taken at a depth of 10-15 cm over an area of 30-40 m ². Samples were placed in polyethylene bags to avoid dehydration, kept in a cool box (approximately at 15 °C) and transported to the laboratory. The hand shovel was sterilized with 70% ethanol after each sampling.

EPN were recovered from the soil samples using Galleria traps method described by Bedding and Akhurst ( 1975). Subsamples, taken from each soil sample, were placed into a clean plastic box with 8 individuals of G. mellonella last instar. The boxes were covered by lids allowing air flow, turned upside down every 24 h to allow up and down free movement of the wax moth larvae inside the plastic boxes and raise their chance to meet nematode infective juveniles (IJs). The boxes were kept in the dark at 25 °C for 1 week. The Galleria larvae were checked at 3-day intervals for the presence of the dead larvae, and insect cadavers were placed individually to modified White traps (White 1927) for the emergence of the infective-stage juveniles. Emerging IJs within the first week were cleaned by distilled water and each nematode isolate was tested against 10  G. mellonella larvae to confirm Koch's postulates for pathogenicity (Kaya and Stock 1997). The juveniles emerging from newly inoculated larvae were maintained on G. mellonella larvae, with regular inoculation each month and stored at 11 °C after rinsed three times with sterile distilled water.

The morphometric assessments of IJs and adults were made using a drawing camera (camera lucida) (Axioplan Axiophoto) attached to a ZEISS (MC100 SPOT) light microscope based on the criteria suggested by Stock and Kaya ( 1996), Hominick et al. ( 1997), and Nguyen and Hunt ( 2007). For morphometric characterization of each isolate, 20 first-generation hermaphrodites of Heterorhabditis species and 20 IJs and 20-s-generation males of Steinernema species were selected at random by dissecting different G. mellonella cadavers after 2-4 and 5-7 days post-infection, respectively. Selected specimens of each isolate were killed by heat, fixed (Seinhorst 1959) in TAF solution (triethanolamine formalin), and transferred to glycerin to mount on slides (Kaya and Stock 1997).

The following taxonomic characters were measured: (L) total body length; (W) maximum body diameter, (ABD) anal body diameter, (EP) excretory pore position, (ES) distance from anterior end to base of pharynx, (NR) distance from anterior end to nerve ring position, (T) tail length, a (total body length divided by maximum body diameter); b (total body length divided by distance from anterior end to base of pharynx); c (body length divided by tail length); (SL) the length of the spicules, (GU) length of the gubernaculum, D% (excretory pore position divided by distance from anterior end to base of pharynx); E% (excretory pore position divided by tail length). After morphological characteristics were made, isolates were placed into related species-groups.

Molecular characterization of each isolate was performed by ITS rDNA (internal transcribed spacer) sequences. DNA extraction was performed, following the protocol of Waeyenberge et al. ( 2000). Newly harvested live 5-10 IJs of each population were used for DNA extraction. The ITS region of the isolates rDNA was amplified by a total of 50 μl PCR reaction mixture containing of 2 μl DNA, 32 μl ddH2O, 15 μl of Dream Taq PCR Master Mix (Fermentas Life Sciences, Germany), 0.5 μM of forward primer 5′-CGTAACAAGGTAGCTGTAG-3′ and 0.5 μl of the 5′-TCCTCCGCTAAATGATATG-3′ (Joyce et al. 1994). The PCR products were sequenced in a sequencing facility in both directions at Refgen, (Ankara, Turkey) and the amplified fragments of Heterorhabditis and Steinernema isolates (Rhabditida: Heterorhabditidae and Steinernematidae) were compared to the sequences of the Heterorhabditis and Steinernema species available in GenBank (NCBI).

A dose-response bioassay was performed by the isolates obtained against last instar larvae of G. mellonella to evaluate their virulence in a Petri dish arena. Ten G. mellonella larvae were placed in sterile 9-cm Petri dishes including 2 moist filter papers (100 × 15 mm) and exposed to IJs suspended in 1 ml of sterile water containing concentrations of 25, 50, and 100 IJs. The Petri dishes applied at different concentrations of IJs were put in a dark incubator at 25 ± 1 °C, R.H. 60%, and only sterile water was applied to the Petri dishes used for control treatments. The bioassay was replicated 3 times and mortality rates were recorded first and second-day post-treatment. Dead larvae were removed from the Petri dishes, incubated individually on modified White Traps at 25 ± 1 °C and dissected to confirm the presence of nematodes.

The data obtained were analyzed by factorial analysis of variance (ANOVA) using SPSS (Version 11.0) statistical software package and Tukey's multiple range test ( P  < 0.05) was used to separate mean values.

Results and discussion

EPNs were isolated from 20 of the 112 soil samples collected from the districts of Hacibektas, Urgup, Avanos, Kozakli, Gulsehir, Acigol, Derinkuyu, and Nevsehir Center with a (17.9%) recovery rate. No nematodes were isolated from the soil samples collected from Gulsehir and Hacibektas districts. Urgup District had the highest proportion of positive soil samples (35%) among others. Vegetable habitats demonstrated a higher recovery frequency (35%) than other habitats and 12 soil samples collected from the forest habitats yielded no nematodes (Table 1).

Table 1 Habitat types and percentage of sites in which entomopathogenic nematode isolates were recovered at Nevsehir in 2015 and 2016

Most of the nematode isolates belonged to the genus Steinernema spp., which were isolated from 18 (90%) out of 20 positive soil samples. The most common species was Steinernema feltiae, which was recovered from 17 soil samples (85%), followed by Heterorhabditis bacteriophora, found only in 2 soil samples (10%) and unidentified Steinernema sp. found only in one soil sample (5%) according to the morphometrical and molecular examination of the isolates (Tables 2 and 3).

Table 2 Entomopathogenic nematode isolates with habitat characteristics of positive sampling sites and ITS sequence information

Table 3 Morphometrics of some of the isolates of Heterorhabditis and Steinernema species obtained (μm)

The virulence of the obtained isolates was evaluated on G. mellonella in Petri dish experiments. The results indicated that all isolates were effective and caused significant mortality rates ranging between 13 and 100% on the first day and 30 and 100% on the second day post-treatment at 25 °C. Larval mortality increased as the concentration of the isolates tested gradually increased and no mortality occurred in control treatments. The maximum mortality rate (100%) on G. mellonella larvae was achieved by S. feltiae DDKB-17 isolate at all concentrations and exposure times, except on the first day at the concentration of 25 IJs. Four of the isolates (UTP-5, UMK-7, AVB-15, and DDKB-17) generally caused higher mortality rates than others at all concentrations and exposure times (Figs. 1 and 2).

The mortality rate (%) of entomopathogenic nematode isolates on Galleria mellonella larvae on the first day after treatment at 25 °C (25, 50, and 100 IJs). UKK-1, MKB-2, UCK-3, KBC-4, UTP-5, DTK-6, MCB-8, UCY-9, DTF-10, DDKY-11, MAY-12, ATB-13, UCB-16, DDKB-17, DME-18, UIP-19, and DMB-20: Steinernema feltiae isolate, UMK-7 and AVB-15: Heterorhabditis bacteriophora isolate, MBB-14: Steinernema sp

The mortality rate (%) of entomopathogenic nematode isolates on Galleria mellonella larvae on the second day after treatment at 25 °C (25, 50, and 100 IJs). UKK-1, MKB-2, UCK-3, KBC-4, UTP-5, DTK-6, MCB-8, UCY-9, DTF-10, DDKY-11, MAY-12, ATB-13, UCB-16, DDKB-17, DME-18, UIP-19, and DMB-20: Steinernema feltiae isolate, UMK-7 and AVB-15: Heterorhabditis bacteriophora isolate, MBB-14: Steinernema sp

The present study is the first comprehensive survey that indicates the common presence and wide distribution of EPNs in the Cappadocia Region, Nevsehir. During the survey, 20 EPN isolates were obtained by a recovery rate of (17.9%) from 112 soil samples collected from 8 districts throughout the province. The recovery rate of EPN isolates (17.9%) was similar to that reported at Karaman province (19.2%) (Yavuzaslanoglu et al. 2016) but remarkably higher than other surveys having recovery rates of (4.71%) in Rize (Ozer et al., 1995), 9% at Adana and Kahramanmaras provinces (Canhilal et al. 2016), 2.03% throughout Turkey (Hazır et al. 2003a). Recovery rates recorded for other European countries in sub-tropical regions were; 13.8% in Southern Italy (Tarasco and Triggiani 2016), 9.5% in Egypt (Shamseldean and Abd-Elgawad 1994) and 4.6% in Spain (Del Pino and Palomo 1996). The reason behind a high recovery rate in the present study might be due to the fact that the soil samples were collected from a narrow area of land at the most appropriate time, especially after rainfall as soil moisture and temperature are among the most crucial factors in the survival of EPNs in the soil environment (Wright 1992; Gaugler and Kaya 1990 and Ehlers and Peters 1996). Of the isolates obtained, the most prevalent ones were S. feltiae (Filipjev 1934) with the ratio of (85%), while the occurrence of H. bacteriophora (Poinar, 1985) (10%), and Steinernema sp. (5%) were rare. This is in agreement with the earlier studies. S. feltiae has been found as the most common EPNs species isolated in the other surveys conducted in Turkey, followed by H. bacteriophora (Laznik et al. 2009; Canhilal et al. 2016 and Canhilal et al. 2017). Both of these EPNs are widely distributed throughout the world. Even though S. feltiae has been found in different climatic zones, it is one of the species that is well-adapted to cold and continental climate conditions (Hazir et al. 2001). When considering the climate of Nevsehir, as continental and its distance from coastal areas, it is an expected result for S. feltiae to be the most isolated EPN species. Although H. bacteriophora is known to be a species that is more adapted to tropical and subtropical areas (Grewal et al. 1994), it has been isolated from many areas with different climatic conditions (Griffin et al. 1999). This suggests that EPNs are distributed in varying frequencies on earth (Griffin et al. 1999).

All the isolates tested had the ability to kill Galleria larvae, but their pathogenicity differed remarkably among different species and/or isolates. In the present study, mortality rates varied between 63 and 100% among the isolates at the concentration of 100 IJs/ml after 48-h exposure time. Similar mortality rates ranging between 78 and 100% were found at the concentration of 100 IJ/ml after 72 h in another study (El Khoury et al. 2018). Tarasco and Triggiani ( 2016) reported that the mortality rates of G. mellonella larvae varying between 78 and 100%, when exposed to different isolates of S. feltiae and H. bacteriophora, following 72 h of exposure to 100 IJs/ml at 25 °C. In this study, the most virulent steinernematid isolate ( S. feltiae, DDKB-17) caused (100%) mortality at the lowest concentration (25 IJ/ml) after 48 h of nematode exposure. Considering the application concentration and exposure time, S. feltiae DDKB-17 isolate appeared to be more virulent than other isolates in other studies. Differences in pathogenicity among the EPNs isolates belonging to the same species have been previously well documented (Canhilal 2013 and Tarasco and Triggiani 2016). These differences in pathogenicity may be due to the application techniques used in these studies or the soil habitats of these nematode species, which is one of the most important factors affecting the performance of EPNs isolates.

Conclusions

Obtained results indicate that the EPNs isolates were highly effective against the G. mellonella larvae where the isolates virulent behavior of S. feltiae UTP-5, DDKB-17  S. feltiae, UMK-7 H. bacteriophora, and AVB-15 H. bacteriophora performed better than the other isolates at all concentrations and exposure times. Therefore, they might have a great potential in the biological control of insect pests. The present study provides insight into the distributions of EPNs in Turkey and further studies are needed to determine the pathogenicity of these isolates against major agricultural insect pests for the selection of the appropriate EPNs isolates.

Availability of data and materials

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

References

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Acknowledgements

We thank Prof. Dr. Halil Elekcioğlu (Cukurova University), Assoc. Prof. Dr. Mustafa İMREN (Abant Izzat Baysal University), Abdurrahman Sami KOCA (Abant Izzat Baysal University), and Ece Bortecine KASAPOGLU (Cukurova University) for their guidance in morphometric and molecular analysis.

Funding

This study was funded by Project No. FYL-2017-7664 by the office of scientific research projects of Erciyes University. The funders had no role in the design of the study and collection, analysis, interpretation of data, and in writing the manuscript.

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Erciyes University, Kayseri, Turkey

Ebubekir Yuksel & Ramazan Canhilal

Contributions

EY and RC conceived and designed the research. EY conducted the experiments, analyzed the data, and wrote the manuscript. Both authors read and approved the manuscript.

Corresponding author

Correspondence to Ebubekir Yuksel.

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Yuksel, E., Canhilal, R. Isolation, identification, and pathogenicity of entomopathogenic nematodes occurring in Cappadocia Region, Central Turkey. Egypt J Biol Pest Control 29, 40 (2019). https://doi.org/10.1186/s41938-019-0141-9
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Keywords

Entomopathogenic nematodes

Heterorhabditis bacteriophora

Soil nematodes

Steinernema feltiae

Survey

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Isolation, identification, and pathogenicity of entomopathogenic nematodes occurring in Cappadocia Region, Central Turkey - Egyptian Journal of Biological Pest Control

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A survey for naturally occurring entomopathogenic nematodes (EPNs) was conducted in various agricultural fields in central Turkey, Nevsehir, between 2015 and 2016 years. EPNs were recovered from 20 of 112 soil samples (17.9%). Seventeen isolates were identified as Steinernema feltiae (Rhabditida: Steinernematidae). One was unknown Steinernema sp.

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Plant parasitic nematode inside the root

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Plant parasitic nematode inside the root

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Plant parasitic nematode inside the root

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Plant parasitic nematode inside the root

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Lambert, K. and S. Bekal. 2002. Introduction to Plant-Parasitic Nematodes. The Plant Health Instructor. DOI: 10.1094/PHI-I-2002-1218-01
Revised 2009 by the Education Center Editorial Board

Kris Lambert and Sadia Bekal
University of Illinois, Department of Crop Sciences, Urbana, IL

The History of Nematodes

Members of the phylum Nematoda (round worms) have been in existence for an estimated one billion years, making them one of the most ancient and diverse ty​​pes of animals on earth (Wang et al. 1999). They are thought to have evolved from simple animals some 400 million years before the "Cambrian explosion" of invertebrates able to be fossilized (Poinar 1983). The two nematode classes, the Chromadorea and Enoplea, have diverged so long ago, over 550 million years, that it is difficult to accurately know the age of the two lineages of the phylum (Figure 1).

Nematodes are multicellular animals in the group Ecdysozoa, or animals that can shed their cuticle. Also included in this group with nematodes are insects, arachnids and crustaceans. In contrast to some of their relative invertebrates, nematodes are soft-bodied. Thus, very few nematodes have been fossilized (22 species from 11 genera) and exactly what ancestral nematodes looked like remains unknown. While we do not know the morphology of the first nematodes, it is probable that they were microbial feeders in the primordial oceans. The oldest known fossil nematodes are only 120-135 million years old; by then nematodes had diversified to feed on microbes, animals and plants (Poinar et al. 1994, Manum et al. 1994). The oldest fossil nematodes are found in amber and are commonly associated with insects. This is probably due to the fact that tree sap, which fossilizes to make amber, captures and preserves insects and their associated nematodes much more easily than an animal- or a nematode-infested portion of a plant. Much of what we know about the evolution of nematodes is inferred from the comparative anatomy of existing nematodes, trophic habits, and by the comparison of nematode DNA sequences (Thomas et al. 1997, Powers et al. 1993). Based upon molecular phylogenic analyses, it appears that nematodes have evolved their ability to parasitize animals and plants several times during their evolution (Blaxter et al. 1998). One point is clear; nematodes have evolved to fill almost every conceivable niche on earth that contains some amount of water. Nematodes are extremely abundant and diverse animals; only insects exceed their diversity. Most nematodes are free-living and feed on bacteria, fungi, protozoans and other nematode (40% of the described species); many are parasites of animals (invertebrates and vertebrates (44% of the described species) and plants (15% of the described species) .

Nematodes were noted early in human history because some serious human diseases are caused by relatively large vertebrate-parasitic nematodes. Some of these nematodes were first described in the ancient Chinese scientific literature as early as 2700 B.C. (Maggenti 1981). Since plant parasitic nematodes often are small and subterranean, there are not many ancient references to phytoparasitic nematodes. One interesting observation suggests that phytoparasitic nematodes were known in antiquity (235 B.C.) because the ancient Chinese symbol for a soybean root-infesting organism resembles in shape an adult female soybean cyst nematode (Noel, 1992). The first described plant parasitic nematodes were discovered in wheat seeds by Needham (1743). Not until the identification of root-knot nematodes on cucumber by Berkeley (1855) and cyst nematodes causing "beet-tired" disease on sugar beets by Schacht (1859), did plant nematology begin to emerge as an important scientific discipline. Nathan A. Cobb, the "father of US nematology," pioneered agricultural nematology as a USDA scientist in the early 1900's. The use of soil fumigation to reduce nematode populations and increase crop yields in the 1940's (Carter) demonstrated that nematodes were significant crop pathogens and ushered in the "chemical era" for nematode management in production agriculture. For a review of the history of plant nematology see the book "General Nematology "by Armand Maggenti (1981); see also the nematode history web sites in Table 1. Today plant parasitic nematodes are recognized as major agricultural pathogens and are known to attack plants and cause crop losses throughout the world. Some estimates suggest they cause 77 billion dollars of damage worldwide each year (Sasser and Freckman 1987). As the full extent of damage caused by plant-parasitic nematodes is recognized by agricultural scientists, the study of the biology of plant-parasitic nematodes will become increasingly important.

Nematode Biology

The study of nematode biology has led to a dramatic increase in understanding of how all animals function. In fact, the bacterial-feeding nematode, Caenorhabditis elegans, is one of the best-understood animals on earth. The fate of every cell in C. elegans development has been carefully mapped. It was the first animal to have its DNA sequence completely deciphered, and it is amenable to detailed genetic analysis. The study of C. elegans has led to new insights into the details of animal development, neurobiology, behavior, and has been of great value in biomedical research as well as in the understanding of nematode biology (Riddle et al. 1997). For more information on C. elegans, also see the web sites listed in Table 1.

The reason nematodes are so useful for biological research is due to their simple anatomy and transparent bodies. Nematodes are simple animals, often only containing 1000 cells or less. Nematodes in all or part of their life cycle are worm-shaped (vermiform), although some species become swollen and rounded in later life stages (). The basic body plan of a nematode is a tube within a tube (). They have an outer skin or cuticle that is secreted from an inner hypodermis. The muscles are attached longitudinally to the nematode's hypodermis, allowing them to move only in the dorsal ventral direction (snake-like movement). Inside the nematode there is an inner tube, the alimentary canal, which runs inside the nematode from head to tail. Between the alimentary canal and the body wall is fluid that provides pressure against the wall to maintain body shape and allow movement. At the head of a plant-parasitic nematode is a hollow mouth spear (like a hypodermic needle) called a stylet (). The nematode uses this stylet to puncture plant cells, to withdraw food and also to secrete protein and metabolites that aid the nematode in parasitizing the plant. The stylet is connected to the pharynx that, in turn, is connected to the intestine. The intestine ends at the rectum in the female nematode and the cloaca in the male. Attached to the pharynx are three - five salivary glands which produce secretions that may be emitted from the stylet and that assist the nematode in plant invasion and parasitism.

The nematode pharynx is muscular and specialized areas can contract and expand the esophageal lining. The expansion and contraction of the pharynx muscles allow the nematode to pump food into its intestine through its stylet or eject secretions from its salivary glands into and around plant cells. See nematode feeding film clips at the "nematodes feeding" web site in Table 1. In rhabditidan nematodes, the esophageal muscles are localized in a muscular organ called the metacorpus (). In enoplean nematodes, the esophageal muscles are more spread out and do not form a compact pumping organ.

In the middle to posterior of the nematode are the reproductive organs. Nematode species often have both males and females, but it is not uncommon for plant nematodes to reproduce asexually by parthenogenesis. In females the reproductive organs are used as traits for identification because the number of ovaries and the position of the vulva in the female nematode's body are easily seen under the light microscope. Male nematodes have one or two testes and they are easily identified by the presence of spicules. Spicules are copulatory structures that are used during mating to guide the sperm into the vagina of the female nematode. See the film clip of soybean cyst nematodes mating at the "SCN mating" web site in Table 2. For a detailed review of nematode structure and function, see "The Structure of Nematodes" by Allan Bird (1991) or the nematode anatomy section of the USDA Nematology Lab web site in Table 1.

The common plant-parasitic nematode genera are fairly easy to identify to that level using a standard compound microscope. Identification of nematodes to the species level often requires detailed morphological analysis, growth of the nematode on different host plants, or DNA or isozyme analysis. Common morphological features used in nematode identification include the mouth cavity (presence or absence and shape of a stylet), the shape and overlap of the pharyngeal glands with the intestine, size and shape of the nematode body at the adult stage, size of the head, tail, and number and position of ovaries in the female. More subtle characters may include number of lines on the nematode's cuticle or the presence or absence of pore-like sensory organs. An excellent guide to nematode identification is "Plant-Parasitic Nematodes: a Pictorial Key to Genera" by William F. Mai et al., 1996.

Plant-parasitic nematodes occur in all sizes and shapes. The typical nematode shape is a long and slender worm-like animal, but often the adult animals are swollen and no longer even resemble worms (). Plant-parasitic nematodes range from 250 um to 12 mm in length, averaging 1 mm, to about 15-35 um in width. While nematodes may look dramatically different, they all share some common features. Nematodes often look segmented because of the numerous annulations (accordion-like transverse grooves) on the cuticle that allow the nematode to bend without kinking (), but in fact nematodes are unsegmented and have no replication of body parts throughout the worm. Like most higher animals nematodes possess bilateral symmetry, but with a superimposed trilateral and hexalateral symmetry. Developmentally, nematodes are triploblastic, containing three body layers (ectoderm, mesoderm and endoderm) in the embryo. Most higher organisms are tripoblastic and have a coelom, a body cavity surrounded by mesoderm. Nematodes have a body cavity that is not totally surrounded by mesoderm, so they are pseudocoelomic.

Nematodes are evolutionarily related to insects, and one feature they have in common is the requirement to molt between juvenile stages. All nematodes undergo four molts from the juvenile to the adult phase of their life cycle (). They have four juvenile stages and an adult stage. In many nematodes the first molt usually occurs in the egg and it is the second-stage juvenile that hatches. While all nematodes undergo four molts, molting is not required for growth of the nematode as it is in insects because of the elasticity of the nematode cuticle. Nematodes do not have a skeleton, but they do have a hypodermis which functions as a flexible support for their muscles. Nematode muscles are arranged along the longitudinal axis of the worm, thus allowing the body of the nematode to move only in a sinusoidal (snake-like) motion. Nematodes have no defined respiratory or circulatory systems; they depend on diffusion of water, gasses and metabolites in and out of their semi-permeable body walls and internal transport by mixing of the pseudocoelomic fluid as the nematode moves. This lack of respiratory/ circulatory systems prevents nematodes from becoming larger in cross sectional area, but does not limit their length. The largest nematode found thus far was more than 7 meter long and 1 cm in diameter. Sorry, no giant nematodes; they are only in the movies. Nematodes have a sophisticated nervous system and sensory organs to help them find their host plant, to locate specific plant cell types, and to mate and reproduce.

Nematode Survival Strategies

One might think of soil as a safe environment, but to a microscopic nematode it is a hostile world filled with danger. A nematode must contend with voracious predators, changes in soil temperature and moisture, and the death of its host plant. For a nematode population to survive, it must be able to circumvent these obstacles. Nematodes evade these biotic and abiotic obstacles by employing a combination of behavioral and physiological survival strategies.

While all nematodes feed on other organisms, the soil is filled with bacteria, fungi and other nematodes that would gladly consume a nutrient-rich plant-parasitic nematode. In fact, the study of nematode predators and pathogens is an important area of study in nematology because nematode predators can be used (in theory) to control populations of plant-parasitic nematodes. While nematodes do possess a thick cuticle that may provide some protection from predation, this type of defense is easily breached by specialized nematode pathogens. The most common method plant nematodes use to evade predation is by living inside plant tissue or by limiting their mobility in the soil environment. By spending less time moving in the soil, a nematode can reduce its chance of "running into" a predator or pathogen. Some plant nematodes spend most of their time in the soil (ectoparasites) and others are mostly contained within the plant tissue (endoparasites). Nematodes that live inside plants have some degree of protection from predation, but they risk death if their host plant succumbs to disease. In contrast, nematodes that move from host to host reduce the risk of perishing with their host, but have a greater chance of encountering a predator or pathogen.

Nematode survival is not impacted only by biotic factors, but also by abiotic ones such as temperature and water availability. The onset of winter or the drying of the soil can be disastrous for a nematode. Interestingly, many nematodes are well adapted to abiotic stress and are capable of cryptobiosis (hidden life): the ability to enter a state of suspended metabolic activity during unfavorable environmental conditions (drying, heat, cold). While not all nematode are capable of cryptobiosis, the ones that are can often survive for years in a cryptobiotic state awaiting favorable conditions that will trigger their revival. The ability of nematodes to undergo cryptobiosis is one reason some nematode species are very difficult to eradicate from a field.

Although cryptobiosis is a useful survival strategy, not all nematodes can enter such a survival stage. Instead, some nematodes have adapted to feed upon many different plant species, thus avoiding the disaster of losing a host plant. Nematodes with a broad host range can feed upon weeds and wild plants making their eradication virtually impossible. By using a complex network of behavioral and biochemical protections nematodes are able to survive and prosper in what would seem to us to be hostile environments. For more information on nematode predators, see Poinar 1983 or the biocontrol web site listed in Table 1. For more information about cryptobiosis see Wharton, 1986.

Nematode Dissemination

While nematodes are motile animals, most are able to move no more than a meter through the soil within their lifetime. However, this lack of long distance crawling does not mean nematodes cannot rapidly spread from field to field. Farm equipment and even muddy shoes contaminated with nematode-infested soil can rapidly disperse nematodes. The movement of water during floods and irrigation can disperse nematodes over long distances. Likewise the movement of nematode infected plants, seeds, and bulbs can give nematodes international tickets to travel the world if plant quarantine officials are not careful. The ability of nematodes to form environmentally resistant stages makes their dissemination even easier, since dried nematodes can be blown with the wind or plant debris over large geographical regions. Even migrating birds are suspected to be able to carry nematodes along their flight paths, assisting the nematodes in their quest for new homes. Essentially any process that moves soil or plant tissue has the ability to disperse plant nematodes, making them difficult plant pathogens to quarantine. For a more detailed explanation of how the soybean cyst nematode has spread throughout the midwestern United States, see Noel, 1992.

Plant-Nematode Interactions

Nematodes feed on all parts of the plant, including roots, stems, leaves, flowers and seeds. Nematodes feed from plants in a variety of ways, but all use a specialized spear called a stylet. Note the differences in stylet length and shape (). The size and shape of the stylet is used to classify nematodes and also can be used to infer their mode of feeding. All three nematodes in Figure 4 are ectoparasites, but Belonolaimus and Longidorus feed deep within the roots using their long stylets, while Helicotylenchus feeds on the exterior of the root or partially burrows into the root to feed using its short stout stylet.

Often nematodes withdraw the contents of plant cells, killing them. When this type of feeding occurs, large lesions are formed in the plant tissue (). Some nematodes do not kill the plant cells they feed upon but "trick" the plant cells to enlarge and grow, thus producing one or more nutrient-rich feeding cells for the nematode. These feeding cells enable long term feeding associations, and form by repeated nuclear division in the absence of cell division (giant cells) or by the incorporation of adjacent cells into a syncytium formed by the breakdown of neighboring cell walls. See Figure 5 for a photomicrograph showing the feeding cells of Meloidogyne (root-knot nematode). Collectively, nematodes can feed on almost any plant cell type, and form a variety of feeding cell types. The number of feeding cells can vary from one to a half dozen depending on the nematode species. For a view of how plant parasitic nematodes move and feed on plants see the film clips and the movies "Wormula" and "The Wormy" at the "nematode movies" web site in Table 1.

Many plant-parasitic nematodes feed on the roots of plants. The feeding process damages the plant's root system and reduces the plant's ability to absorb water and nutrients. Typical nematode damage symptoms are a reduction of root mass, a distortion of root structure and/or enlargement of the roots (). Nematode damage of the plant's root system also provides an opportunity for other plant pathogens to invade the root and thus further weakens the plant. Direct damage to plant tissues by shoot-feeding nematodes includes reduced vigor, distortion of plant parts, and death of infected tissues depending upon the nematode species.

The aboveground symptoms of nematode damage to roots are relatively nondescript, including nutrient deficiency, incipient wilt, stunting, poor yield and sometimes plant death. Few diagnostic signs and symptoms of plant damage by nematodes exist except root galls, cysts, "nematode wool," and seed galls (see below). Thus, damage to crops by root-infesting nematodes often goes unnoticed by growers. Field patterns of nematode damage to roots begin in a small area and spreads radially from the initial infection site, often assisted by farm equipment. The only way to accurately diagnose nematode disease is to sample soil and plant material from suspected sites and extract nematodes for analysis. Nematodes are extracted from the soil by floating them in water to remove heavy soil particles and then catching the floating nematodes on sieves with fine pore sizes. Plant tissues infected with motile nematodes can be incubated in a Baermann funnel or moist chambers to collect nematodes that will exit the tissues.

Most plant parasitic nematodes are soilborne root pathogens, but a few species feed primarily upon shoot tissues. The majority of plant parasitic nematode species are in the class Chromodorea, order Rhabditida (formerly placed in the order Tylenchida). There are seven major types of nematode feeding strategies used by plant parasitic nematodes (Table 2).

Ectoparasites: The first feeding type is the ectoparasitic mode, in which the nematode remains outside of the plant and uses its stylet to feed from the cells of the plant roots (Figure 9). Nematodes that use this strategy can graze on numerous plants, making it easier for them to switch hosts, but their added mobility makes them very susceptible to environmental fluctuations and predators. Ectoparasitic nematodes can have extremely long stylets, which assist them in feeding deep within the plant root on nutrient rich plant cells (). Some of these nematodes induce the plant to form an enlarged cell(s) that the nematode feeds from for an extended period of time. Note, in all life cycle diagrams in this article the abbreviation J=juvenile and the number refers to the stage of the nematode and M=molt and refers to how many molts the nematode has completed.

The above life cycle is typical for a nematode in the class Enoplea (Figure 9), but most nematodes in the class Chromadorea undergo their first molt in the egg and hatch as J2. All motile nematode stages are capable of feeding from the plant. The nematodes feed, undergo four molts into adults, mate and lay eggs. Some nematodes that use this feeding strategy can form terminal galls in the roots and cause severe stunting of the root system. An example of an ectoparasite is Xiphinema (dagger nematode), shown below in Figure 10, feeding from a fig root. This enoplean nematode is particularly problematic because it can harbor and transmit plant viruses. Only ectoparasitic nematodes in the class Enoplea transmit viruses, but because of the virus transmission, this type of nematode in low numbers can still be very damaging to plants (Fisher and Raski, 1967; Hewitt et al. 1958).

Semi-endoparasites: Nematodes that feed as semi-endoparasites are able to partially penetrate the plant and feed at some point in their life cycle (). Usually the head of the nematode penetrates into the root and allows the nematode to form a permanent feeding cell. These nematodes swell and do not move once they have entered into the endoparasitic phase of their life cycle. By giving up their mobility, the nematodes risk death if their host plant dies, but they also benefit from forming a permanent feed site, which increases their nutrient uptake and reproductive potential. A typical nematode with this life cycle is Rotylenchulus reniformis, the reniform (kidney-shaped) nematode. This nematode hatches from the egg as a J2, then quickly molts in the soil to the adult stage without feeding. The anterior end of an adult female enters the plant root and forms a feeding cell. After mating, the female lays its eggs outside of the root in a gelatinous egg mass Maggenti (1981). Another nematode with a similar feeding strategy is Tylenchulus semipenetrans, the citrus nematode, although the juvenile stages of this nematode do feed as ectoparasites. As is common in biological systems, it is often difficult to precisely classify animals due to variation in their behavior. True to this rule, several species of ectoparasitic nematodes (e.g. Helicotylenchus) are also capable of partially penetrating the root and feeding. However, we do not classify these nematodes as semi-endoparasites because they do not exhibit a consistent endoparasitic feeding behavior.

Migratory endoparasites: Migratory endoparasitic nematodes spend much of their time migrating through root tissues destructively feeding on plant cells (). These nematodes cause massive plant tissue necrosis because of their migration and feeding. When they feed from the plant, they simply suck out the plant cell cytoplasm using their stylet, killing the plant cell and moving ahead of the lesion. They make no permanent feeding cells. In a typical life cycle, the nematode hatches from the egg as a second-stage juvenile and starts feeding on the plant. The nematodes feed, molt and reproduce primarily within the plant tissue. All motile stages are capable of feeding from the plant and they are able to move into the soil in search of new roots to invade. Because these nematodes create extensive wounds in the plant root, secondary infection by bacteria and fungi can often occur, further damaging the root system (Zunke 1991). Below is a typical life cycle for a migratory endoparasite. Examples of migratory endoparasitic nematodes are Pratylenchus (lesion nematode), Radopholus (burrowing nematodes) and Hirschmanniella (rice root nematode).

Sedentary endoparasites: The most damaging nematodes in the world have a sedentary endoparasitic life style. The two main nematodes in this group are the cyst nematodes ( Heterodera and Globodera) and the root-knot nematodes ( Meloidogyne). In these nematodes, the J2 invades the plant near the tip of a root and migrates through the tissue to the developing vascular cells. These nematodes are completely embedded in the root during their initial stages of development, but later the cyst nematodes protrude from the root. The J2 nematodes inject secretions into and around the plant cells to stimulate the formation of large feeder cell(s), which they non-destructively feed on throughout their life cycle. Feeding cells of root-knot nematodes (giant cells) form by repeated nuclear division in the absence of cell division. Feeding cells of cyst nematodes form by the incorporation of neighboring cells into a syncytium formed by the breakdown of neighboring cell walls. Once the feeding cells are formed, the juvenile nematode rapidly becomes sedentary because their somatic muscles atrophy. The juveniles feed, enlarge and molt three times to the adult stage. The large feeding cells formed by these nematodes plug the vascular tissue of the plant making it susceptible to water stress. Female sedentary endoparasites enlarge considerably into a saclike shape and are capable of laying large numbers of eggs. Eggs are typically laid outside the nematode in a gelatinous egg mass, but in cyst nematodes most eggs are retained inside the female body. Both types of nematode have the same basic feeding strategy, but many cyst nematodes have an obligate sexual cycle, whereas the most common species of root-knot nematodes reproduce largely by parthenogenesis.

Below, the life cycle of the soybean cyst nematode ( Heterodera glycines) is described (). The cyst nematodes are extremely problematic because they have the ability to persist for a long period of time in a field. Their remarkable persistence is due to their ability to produce a cyst, which is the hardened dead body of the female nematode that surrounds the eggs. The eggs in the cyst are in a dormant state and they hatch slowly over years in the field, usually in response to signals from host roots. Having a resistant or dormant nematode stage enables nematodes to survive non-optimal conditions. Resistant stages also aid the nematode in dispersal. While the root-knot nematodes do not have an environmentally resistant stage, they do possess a broad host range that enables them to persist on an alternate host. The use of environmentally resistant nematode stages and broad host ranges are two key strategies plant nematodes use to persist in the environment. For further information about the soybean cyst nematode see the "Biology and management of the soybean cyst nematode" edited by R.D. Riggs and J.A. Wrather. For further information about the root-knot nematode see "An advanced Treatise on Meloidogyne" edited by J.N. Sasser and C.C. Carter.

Stem and bulb nematodes: Stem and bulb nematodes ( Ditylenchus spp.) are, as their name suggests, nematodes that attack the upper and lower parts of plants (). They use water films to migrate up the stem of the plant and therefore are more damaging under wet conditions. The infectious stage of the stem and bulb nematodes is the fourth stage juvenile. This stage often enters emerging plant tissues below ground, but can crawl up stems in a film of water and enter shoots via buds, petioles, or stomata. Once in the host plant, they destructively feed as migratory endoparasites, molt into adults and reproduce. The nematodes hatch from the egg as J2 and continue to feed, molt and reproduce, extensively macerating and distorting the plant tissue (). Once the plant is destroyed or winter arrives, the stem and bulb nematode juveniles arrest their development at the environmentally resistant J4 stage and overwinter. Fluffy masses of dried (cryptobiotic) Ditylenchus can be seen on the surface of bulbs and are known as "nematode wool." Once environmental conditions are favorable, the cryptobiotic J4 become active and their life cycle resumes (Hooper 1972).

Bursaphelenchus xylophilus (pine wood nematode), a nematode that infects pine trees, has a different and fascinating life cycle (). Resistant stages of these nematodes are carried into pine tree in the tracheae of bark beetles. Once in the tree the nematodes are released from the beetle, and they migrate through the resin canals of the tree. The pine wood nematode destructively feeds from cells in the resin canals, clogging them and the tree's vascular tissue causing pine wilt disease. The nematodes feed, molt and sexually reproduce at a highly accelerated rate. The population of pine wood nematodes builds up rapidly and quickly kills the host tree. The nematodes are able to feed on fungi in the tree after the plant cells are dead. The nematodes survive the winter as J3s and are attracted to beetle larvae. In the spring the nematodes molt to the resistant J4 stage and invade the beetle to be dispersed to neighboring trees. The ability of the nematode to be dispersed by an insect allows them to spread rapidly from tree to tree, a process which can quickly destroy a forest (Mamiya 1983). This nematode is thought to be indigenous to the Unitied States where all of the native trees are resistant to the damaging effects; however in Asia and Europe, the pine tree species are very susceptible, which has resulted in the death of millions of trees each year.

Seed gall nematodes: Seed gall nematodes ( Anguina spp.) were the first plant-parasitic nematodes to be described in the scientific literature in 1743 (). These nematodes migrate as J2s in water films to the leaves of plants where they feed as ectoparasites at the tips, causing distortion of the leaves. Once the plant starts to flower the J2 penetrates the floral primordia and starts to feed on the developing seed. Once in the seed, the nematode undergoes its molts, continues to feed, and eventually kills the seed to form a blackened "cockle" (seed gall). The adults sexually reproduce, the eggs hatch as J1 and then quickly molt into a J2 survival stage. The environmentally resistant J2 desiccates with the seed gall and overwinters. The nematodes in the seed gall can survive for 30 years if kept in a dry location. When proper moisture and temperature conditions arise, the cryptobiotic J2 becomes active and starts the life cycle over again (Southey 1972).

Foliar nematodes: Foliar nematodes are in the genus Aphelenchoides. The adult nematodes migrate in water films on the stems to the leaves of their host plant and penetrate the leaves through natural openings (stomata) (). Once in the leaves the nematodes migrate, destructively feed, molt and lay eggs. The feeding activity of the nematodes causes characteristic interveinal chlorosis and necrosis of the leaf, ultimately killing it. The nematodes are able to move from leaf to leaf if the proper (moist) environmental conditions exist and can severely damage a plant. In the winter the adult nematodes persist in the dead leaves until favorable conditions arise in the spring. If the dead, nematode-infested leaves are moved or blown around this will help disperse the nematode near new host plants (Hesling and Wallace 1961).

Plant Nematode Control

There are several methods commonly used to control plant-parasitic nematodes. These methods can be divided in to three main types: biological control, cultural control and chemical control. The most practical form of biological control is the use of nematode-resistant plants. In this control method, plant breeders cross natural nematode resistance genes into cultivated plant species to improve their resistance to nematodes. The benefit of this method is that it is a very inexpensive way for growers to control their nematode problems. The main disadvantage is that it takes years to screen for resistant plant varieties and more time to breed resistance traits into commercial varieties. Further complications are that natural sources of nematode resistance do not exist for all cultivated species and some species of nematodes are able to grow on resistant plants. However, when "good" resistant plants are available, they are an effective method of nematode control. To find a list of plants resistant to nematodes see the plant host database web site in the Department of Nematology at the University of California at Davis (Table 2). Biotechnology has the potential to produce nematode resistance genes that would be effective against many types of nematodes and would function in many different plant species. However, at the present time there are no plants with bioengineered nematode resistance available for commercial use.

Other biological control methods use natural predators or pathogens of nematodes. While biological control agents kill nematodes in controlled laboratory settings, implementation of biological control methods in the field is not done due to the expense and difficulty of growing large amounts of nematode pathogens. However, if some of the difficulties associated with growing nematode pathogens can be overcome, this may be a promising method of nematode control in the future.

Crop rotation with a non-host plant is a very effective method to limit nematode growth. Typically, a cropping system is devised that selects plants that nematodes can and cannot grow on. These plants are grown in alternate years and the problematic nematode population decreases dramatically, below damage threshold levels, in the years that the non-host is grown. This can be an effective method if a producer has the choice of several different crops that can be grown and if the problematic nematode does not have a broad host range or survive in the soil in a cryptobiotic state for long periods of time. See the UC Davis nematode databases in Table 2 to ascertain the host range of various plant parasitic nematodes.

For the past 50 years nematodes have been effectively controlled using chemical nematicides. These are inexpensive chemicals that effectively kill nematodes in soil. There are two types of nematicides, soil fumigants (gas) and non-fumigants (liquid or solid). Soil fumigants became popular because they did not rely on alternative host crops for rotation; they drastically reduced nematode populations in the soil, and were cost effective for most crops. Most fumigant nematicides have been banned by the EPA as environmental toxins with the exception of 1,3 dichloropropene (Telone II), chloropicrin (tear gas), and dazomet (Basamid). The multipurpose soil fumigant methyl bromide also provides excellent reduction of soil nematode populations, but methyl bromide was largely discontinued in 2005. Non-fumigant nematicides such as fenamiphos (Nemacur) and aldicarb (Temik) are based upon the same kinds of active ingredients as many insecticides (i.e. nerve poisons) and can be applied in liquid or granular formulations. While non-fumigant nematicides reduce nematode populations, their effectiveness is not as consistent as that of fumigant nematicides. The EPA is also restricting the use of non-fumigant nematicides. Since nematicides are expensive to develop, new ones are rarely released on the market today. While nematicides are effective in controlling nematodes, they are only practical for use on high-value crops.

Plant nematodes are not typically controlled using just one method mentioned above, but instead they are managed using a combination of methods in an integrated pest management system. For more detailed information about nematode control see "Plant Nematode Control" by A.G. Whitehead 1998 or the web site for chemical methods of nematode control in Table 1.

* eggs, all juvenile stages and adults can survive the winter, but not egg producing females.

Other APS Education Center nematode links

K-12 Exercise Tylka, G.L. and C.A. Jasalavich. 2001. Nematodes: Plant-Parasitic Worms.

References

Barker, K.R., G.A. Pederson, and G.L. Windham. 1998. Plant and Nematode Interactions. ASA, CSSA, and SSSA, Madison, WI.

Bird, A.F. and J. Bird. 1991. The Structure of Nematodes, Second edition.Academic Press Inc. London.

Blaxter, M.L., P. De Ley, J.R. Garey, L.X. Liu, P. Scheldeman, A. Vierstraete, J.R. Vanfleteren, L.Y. Mackey, M. Dorris, L.M. Frisse, J.T. Vida, K.W. Thomas, K.W. 1998. A molecular evolutionary framework for the phylum nematoda. Nature 392:71-75.

Campbell, J.F. and H.K. Kaya. 1999. How and why a parasitic nematode jumps. Nature 397: 485-486.

Chen, Z.X., and D.W. Dickson. 2004. Nematology: Advances and Perspectives Vol. 1: Nematode Morphology, Physiology and Ecology. CABI:Walllingford.

Chen, Z.X., S.Y. Chen, and D.W. Dickson. 2004. Nematology: Advances and Perspectives Vol. II: Nematode Management and Utilization. CABI:Wallingford.

Fisher, J.M. and D.J. Raski. 1967. Feeding of Xiphinema index and X. diversicaudatum. Proceeding of the Helminthological Society of Washington 34:68-72.

Hewitt, W.B., D.J. Raski, and A.C. Goheen. 1958. Nematode vector of soil-borne fanleaf virus of grape vine. Phytopathology 48:586-595.

Hesling, J.J. and H.R. Wallace, H.R. 1961. Observations on the biology of chrysanthemum eelworm Aphelenchoides ritzemabosi (Schwartz) Steiner in florists chrysanthemum. I. Spread of eelworm infestation. Annals of Applied Biology 49:195-209.

Hooper, D.J. 1972. Ditylenchus dipssaci. Commonwealth Institute of Helminthology Descriptions of Plant Parasitic Nematodes, Set 1, No. 14, St. Albans, England.

Lee, D.L. 2002. The Biology of Nematodes. Tayor & Francis; New York

Maggenti, A. 1981.General Nematology. Springer-Verlag, New York, NY.

Mai, W.F., P.G. Mullin, H.H. Lyon, K. Loeffler. 1996. Plant-parasitic nematodes: a pictorial key to genera. Cornell University Press, Ithaca, NY.

Mamiya, Y. 1983. Pathology of the pine wilt disease caused by Bursaphelenchus xylophilus. Annual Review of Phytopathology 21: 201-220.

Manum, S.B., M.N. Bose, R.T. Sayer, R.T., S. Bostrom. 1994. A nematode (Captivonema-Cretacea Gen ET SP-N) preserved in a clitellate cocoon wall from the Early Cretaceous. Zoologica Scripta. 23: 27-31.

Perry, R.N. and M. Moens. 2006. Plant Nematology. CABI: Wallingford.

Noel, G.R. 1992. History, Distribution, and Economics. Pages 1-13 in: Biology and Management of the Soybean Cyst Nematode. R.D. Riggs and J.A. Wrather, eds. American Phytopathological Society, St. Paul, MN.

Poinar, G.O., A. Acra, and F. Acra. 1994. Earliest fossil nematode (Mermithidae) in Cretaceous Lebanese amber. Fundamental and Applied Nematology 17:475-477.

Poinar, G.O. 1983. The Natural History of Nematodes. Prentice Hall, Englewood Cliffs, NJ.

Powers, T.O., T.S. Harris, and B.C. Hyman, B.C. 1993. Mitochondrial DNA sequence divergence among Meliodogyne incognita, Romanomermis culicivorax, Ascaris suum, and Caenorhabditis elegans. Journal of Nematology 25:564-572.

Riddle, D.L., T. Blumenthal, B.J. Meyer, J.R. Priess (eds). 1997. C. elegans II. Cold Spring Harbor Monograph Series 33. Cold Spring Harbor Laboratory Press. Plainview, NY.

Riggs, R.D., and J.A. Wrather. 1992. Biology and Management of the Soybean Cyst Nematode. American Phytopathological Society, St. Paul, MN.

Sasser, J.N. and C.C. Carter. 1985. An advanced Treatise on Meloidogyne. North Carolina State University Graphics, Raleigh, NC.

Sasser, J.N. and D.W. Freckman. 1987. A world perspective on nematology: the role of the society. Pages 7-14 in: Vistas on Nematology, J.A. Veech and D.W. Dickson, eds. Society of Nematologists, Inc. Hyattsville, MD.

Siddiqi, M.R., 2000. Tylenchida. CABI: Wallingford.

Shurtleff, M.C., and C.W. Averre. 2000. Diagnosing Plant Diseases Caused by Nematodes. APS Press: Minneapolis

Southey, J.F. 1972. Anguina tritici. Commonwealth Institute of Helminthology Descriptions of Plant Parasitic Nematodes, Set 1, No. 13, pp.1-4. St. Albans, England.

Starr, J.L., J. Bridge, and R. Cook. 2002. Plant resistance to parasitic nematodes. Oxford University Press; Oxford

Thomas, K.W., J.T. Vida, L.M. Frisse, M. Mundo, J.G. Baldwin. 1997. DNA sequence from formalin-fixed nematodes: integrating molecular and morphological approaches to taxonomy. Journal of Nematology 29:250-254.

Viglierchio, D.R. 1991. The World of Nematodes. AgAccess, Davis, CA.

Wang, D.Y.-C., S. Kumar, and B.S. Hedges. 1999. Divergence time estimates for the early history of animal phyla and the origin of plants, animals and fungi. Proceedings of the Royal Society of London B 266: 163-171..

Wharton, D.A. 1986. A Functional Biology of Nematodes. The John Hopkins University Press, Baltimore, MD.

Whitehead, A.G. 1998. Plant Nematode Control. CAB International, New York. NY.

Zunke, U. 1991. Observations on the invasion and endoparasitic behavior of the root lesion nematode Pratylenchus penetrans. Journal of Nematology 22:309-320.

​

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Education Center. Introduction to Pathogen Groups....Lambert, K. and S. Bekal. 2002. Introduction to Plant-Parasitic Nematodes. The Plant Health Instructor. DOI: 10.1094/PHI-I-2002-1218-01Revised 2009 by the Education Center Editorial Board Kris Lambert and Sadia BekalUniversity of Illinois, Department of Crop Sciences, Urbana, ILThe...

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By Sheri Bethard

Orange County Master Gardeners, Adapted from Social Science Society of America blog

It might be hard to believe one of the most abundant animals on Earth is - soil nematodes! There are thousands of nematode species with most not being harmful. They represent 80% of animal life by number and live in nearly every habitat. Some cause diseases in humans (trichinosis) and animals while others feed on living plants. Nematodes normally get a bad reputation. But little is known about the non-parasitic nematodes, predatory and microbial-feeding, which have many beneficial roles.

They are hard-working, important, translucent, unsegmented, very tiny organisms' (worm-like) - usually between 1/500 th to 1/20 th of an inch! They are not true worms or in the worm species. Interesting Fact: There is a nematode that lives inside sperm whales that is nearly thirty feet long.

Some of the best-known nematodes are parasites (Root Knot). Some nematodes require a host to live, while many more nematodes are free-living in soils in water films that surround the soil particles.

There are three types of nematodes - parasitic, microbial-feeding, and predatory. The parasitic nematodes are bad nematodes while the microbial-feeding and predatory nematodes are mostly good nematodes. Plant-root parasitic nematodes harm plant growth and microbial-feeding nematodes improve it, but other nematodes are also important. For example, predatory nematodes play an important role in regulating populations of plant-parasitic and microbial-feeding nematodes. Predatory feeding hel keep populations of plant parasites and microbial feeding nematodes in check, optimizing plant growth.

Some nematodes feed on plants and algae, others graze on microbes (bacteria and fungi). At higher levels in the food web, nematodes that are predators and omnivores eat other nematodes and invertebrates. By feeding on bacteria and fungi, microbial-feeding nematodes return nitrogen to the soil through their waste, making nitrogen available again for plant use, improving plant growth.

As nematodes move around in soil, they deposit bacteria in new places, spreading it around. The bacteria can contribute to and speed the process of decomposition, returning carbon to the soil for storage.

But most good things have a limit: at very high populations, nematodes that feed on bacteria and fungi can reduce their populations. This can lead to lower decomposition and nutrient turnover rates and lowering plant growth.

Plant parasitic nematodes attack roots using a piercing tool in their mouth. This "stylet" punctures plant cells so it can suck its carbon-rich juices. Some nematodes release chemicals that cause lesions or tumor-like growths on roots, thus the knots you see. They drain the plant's strength above and below ground. Plants most attacked at the roots include tomatoes, potatoes peppers, lettuce, corn and carrots.

However, predatory (good) nematodes are also highly sensitive to environmental changes. Their populations often decline with soil disturbances such as pesticide use, fertilization, tilling, or soil compaction. Situations where soil is heavily managed often leads to very low predator populations and higher populations of harmful groups. Predators are also sensitive to changes in rain and temperature, which can also cause an imbalance toward harmful groups.

In small populations, plant parasitic nematodes can stimulate root growth, but in high numbers they destroy roots, stunt above ground growth, and cause disease.

While the nematode species responsible for plant diseases have received a lot of attention, far less is known about the non-parasitic part of the soil nematode community, which plays mostly beneficial roles in soil. Ensuring a balance between beneficial and plant parasitic nematode groups is important for plant health and its contributions to soil carbon.

Research is examining how active land management practices, such as adding compost, which directly provides nutrients, increases soil water retention, improve plant growth and soil habitat for predators will support them to help keep root parasitic nematode populations in check. As plant disease and root parasitism declines, it leadins to greater plant growth and root carbon inputs benefiting soil carbon storage. Nematodes require moist, humid conditions and warm soil.

To battle harmful nematodes, purchase plants and seed labeled nematode-resistant, and consider solarizing your soil during the summer prior to planting. In addition, you can purchase beneficial nematodes for your garden. Buy from a reputable supplier, such as Rincon Vitova, Biologic, and others may be found online. Some of the big box stores have started carrying them also. Nematodes are perishable and shipping stresses them and could even kill many of them if not packaged correctly or shipping takes to long. It is recommended to use 13,000-35,000 nematodes per square foot.

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How do nematodes help plants and soils? - Picayune Item

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By Sheri Bethard Orange County Master Gardeners, Adapted from Social Science Society of America blog It might be hard to believe one of the most abundant animals on Earth is - soil nematodes! There are thousands of nematode species with most not being harmful. They represent 80% of animal life by number and live in [...]

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While most of the thousands of nematode species on Earth are not harmful, some cause diseases in humans and other animals or attack and feed on living plants. Luckily, there are ways to deter these pesky pests from disrupting your garden soil.

The few parasitic species of these translucent, unsegmented worms measure about 1/50 inch long and cause root knots or galls, injured root tips, excessive root branching, leaf galls, lesions or dying tissue, and twisted, distorted leaves. Plants most commonly attacked at the roots include cherry tomatoes, potatoes, peppers, lettuce, corn, and carrots. Plants that sustain leaf and stem system injury include chrysanthemums, onions, rye, and alfalfa.

What Are Nematodes?

Often referred to as roundworms, nematodes are not closely related to true worms. They are multicellular insects with smooth, unsegmented bodies. The nematode species that feed on plants are so tiny that you need a microscope to see them. The adults often look long and slender, although some species appear pear-shaped. These plant parasites are not the same roundworms as the filarial nematodes that infect the human body, spread diseases, and wreak havoc on the immune system.

Some nematodes feed on the outer surfaces of a plant while others burrow into the tissue. Soil-dwelling nematodes are the most common culprits, but some species can damage plant roots, stems, foliage, and flowers.

No matter where they feed, these tiny worms can seriously damage to crops with their sharply pointed mouths by puncturing cell walls. The real damage occurs when a nematode injects saliva into a cell from its mouth and then sucks out the cell contents. The plant responds to the parasitic worms with swelling, distorted growth, and dead areas. Nematodes can also carry viruses and bacterial diseases inject them into plants. The feeding wounds they make also provide an easy entrance point for bacteria and fungi.

Beneficial nematodes that enrich the soil may feed on the decaying material, insects, or other nematodes.

What Nematodes Look Like

Unlike most other disease-causing organisms, plant-parasitic nematodes seldom produce any characteristic symptoms. Most of the symptoms that do appear are vague and often resemble those caused by other factors - such as viruses, nutrient deficiencies, or air pollution. Nematodes feeding aboveground may cause twisted and distorted leaves, stems, and flowers.

Root nodules invaded by nematodes.

If nematodes are feeding on the roots, a plant may look yellowed, wilted, or stunted and infected food crops will usually yield poorly. If you suspect worm injury to roots, carefully lift one of the infected plants and wash off the roots for easier inspection. If nematodes are causing damage, you may see small galls or lesions, injured root tips, root rot, or excessive root branching.

How They Spread

Whether they feed above or below ground, most nematodes spend at least part of their life cycle in the soil. While they can't move very far under their own power, they can swim freely in water and they move more quickly in moist soil - so it's a good idea to keep your soil well-drained. They also spread by anything that can carry particles of infested soil, including tools, boots, animals, and infected plants.

What About Beneficial Nematodes?

Beneficial nematodes can range from 1/25 inch to several inches long and have slender, translucent, unsegmented bodies. Their roles in the garden vary. Some are soil dwellers that break down organic matter, especially in compost piles. You can easily spot these 1/4-inch-long decomposers.

These types actually combat a variety of pest species, including weevils, clearwing borers, cutworms, sod webworms, chinch bugs, and white grubs. Nematodes attack and kill these insects by either injecting deadly bacteria or entering the host, parasitizing, and then feeding on it.

When purchasing and applying them to your garden, it is very important to select the right species because different kinds of nematodes are effective against different pests. In addition, nematodes require moist, humid conditions and fairly warm soil to do their job well. Water all application sites before and after spreading nematodes and follow application instructions carefully.

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What Are Nematodes? These Tiny Parasites Can Help - Or Hurt - Your Garden

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While most of the thousands of nematode species on Earth are not harmful, some cause diseases in humans and other animals or attack and feed on living plants. Luckily, there are ways to deter these pesky pests from disrupting your garden soil.

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Top 10 plant-parasitic nematodes in molecular plant pathology.

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Top 10 plant-parasitic nematodes in molecular plant pathology.

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What Are Nematodes? Nematode Under A Microscope

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https://www.youtube.com/watch?v=ZrogAKO3dhI

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Cyst nematode management in soybean cultivation

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Cyst nematode management in soybean cultivation

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https://apsjournals.apsnet.org/doi/pdfplus/10.1094/PD-89-1020

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The impact of plant-parasitic nematodes on agriculture and methods of control

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The impact of plant-parasitic nematodes on agriculture and methods of control

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START FROM PAGE 121:

https://books.google.com.my/books?hl=id&lr=&id=MQWQDwAAQBAJ&oi=fnd&pg=PA121&dq=NEMATODES+AS+PLANT+DISEASE&ots=bCd6Evmfmf&sig=QzvYoTTiQkJKxsZpFT_qNfM9GXI&redir_esc=y#v=onepage&q=NEMATODES%20AS%20PLANT%20DISEASE&f=false

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Symptom and injury to plant caused by nematodes

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In this article we will discuss about:- 1. Introduction to Nematodes 2. Symptoms Caused by Nematodes 3. Mechanism of Nematode Injury to Plants 4. Interrelationship between Nematodes and Other Plant Pathogens 5. Ecology and Spread 6. Characteristics 7. Life Cycle.
Contents:

1. Introduction to Nematodes

2. Symptoms Caused by Nematodes

3. Mechanism of Nematode Injury to Plants

4. Interrelationship between Nematodes and Other Plant Pathogens

5. Ecology and Spread of Nematodes

6. Characteristics of Plant Pathogenic Nematodes

7. Life Cycle of Nematodes

1. Introduction to Nematodes:

Nematodes are the most highly developed of the pseudo-coelomates. They belong to the phylum Nemata, previously named Phylum Nemathelminthes or Phylum Aschelminthes. They are widely present in the soil, fresh water or marine water.

Nematodes are the only plant parasites belonging to the animal kingdom which are studied in plant pathology Nematodes, sometimes called eelworms, are worm-like in appearance but quite distinct taxonomically from the true worms- Numerous species of nematodes attack and parasitize man and animals and cause various diseases. Several hundred species are known to feed on living plants as parasites and cause a variety of plant diseases.

A general account of nematodes is given by Crofton (1966). One of the most interesting works on the biology of plant parasitic nematodes was published by Wallace in 1963. Nematology in India was recently reviewed by Swarup and Seshadri (1974). The methods of controlling nematodes have been discussed by Khan. Nematodes as parasites of plants, their ecology and the process of infection are discussed by Dropkin (1977).

All plant parasitic nematodes belong to the phylum Nemathelminthes, class Nematoda. Most of the important parasitic genera belong to the subclass Secementea- Order Tylenchida.

The characteristic features of the subclass Secementea are - Excretory canals (protoplasmic) present, terminal excretory duct cuticularized; caudal glands absent; phasmids usually present; amphids usually minute, pore-like and cephalic in position; sensory organs papilloid, seldom setose; hypodermal glands absent; male with or without caudal alae.

The taxonomi-problems concerning the phytopathogenic nematodes have been reviewed by Allen and Sher (1967). An extraordinary amount of attention has been given, in the last 20 years, to groups that contain economically important phytopathogenic nematodes.

This has been primarily due to the realization that phytopatho genie nematodes are worldwide in distribution and that they are frequently associated with crop diseases and decreased food production.

Till the end of 1966, a total of 1079 species had been described in the taxa that contain the phytopathogenic nematodes. Up to 1950, 46 genera were recognized in the plant parasitic groups and in the period 1950 to 1966 an additional 65 genera were proposed, making a total of 111 genera with phytoparasitic nematodes.

2. Symptoms Caused by Nematodes:

Nematodes infect the roots as well as the parts of the plant which are above the ground.

Root symptoms may appear as hypertrophy, necrosis or abnormal growth and include the following:

1. Root Knots or Root Galls:

These are enlargements of the roots caused by the feeding of the nematodes which may not necessarily be enclosed within them. The swelling may vary in size from 1 mm to more than 2 cm.

The feeding of nematodes induces the formation of 'giant cells' in the host tissue and cell division is stimulated. This leads to the formation of galls of various sorts. Bird (1974) has discussed the response of plants to root knot nematodes under two major headings.

First, the responses of entire plants, and second, the responses of their cells. Whole plants respond to infection by reducing their photosynthetic rate, 'growth and yield. It is suggested that the nematode influences the physiology of the plant by interfering with the synthesis and translocation of growth hormones produced in the roots. Cells of susceptible plants are changed from normal undifferentiated cells to highly specialized syncytia, also known as giant cells or multinucleate transfer cells.

This process involves a chain of events, starting with nuclear and nucleolar enlargement, followed by cell wall breakdown, synchronous mitoses and incorporation of adjacent cells. This highly specialized syncytium is induced and maintained by, and is completely dependent on a continuous stimulus from the nematodes.

2. Root Lesions:

There are discoloured and often collapsed portions of the root, consisting of cells on which nematodes have fed. They vary in size, from being as minute as to be almost invisible to the naked eye to lesions girdling the whole root. Necrotic lesions are probably caused by toxic salivary secretions injected during the feeding of nematodes, for example - Radopholus.

This is caused by the formation of numerous short laterals in the vicinity of nematode injury.

Root rots occur when nematode infections are accompanied by plant pathogenic or saprophytic bacteria and fungi.

This happens when the nematodes feed on or near root tips and cause them to stop growing, enlarging or disintegrating. Nematodes feeding ecto-parasitically at the root tips suppress cell division in the apical meristem and result in short roots, as in Trichodorus, the stubby root nematode.

Nematode root infections are usually accompanied by non- characteristic symptoms in the above-ground parts of plants, appearing primarily as reduced growth, symptoms of nutrient deficiencies, such as yellowing of foliage, excessive wilting in hot or dry weather, reduced yields and poor quality of products.

Nematodes which attack the aerial parts of the plant may cause discolouration, necrosis, blotches, spots, distortion and galls on the leaf, stem and seed. Buds, growing points, or flower primordia are attacked by some nematodes, resulting in the abnormal growth of the affected plant.

These symptoms are thought to result from substances secreted by the eel worm or perhaps by the invaded plant tissue. Krusberg (1963) and Singh (1964) have reviewed the nature of plant reaction to infection by nematodes. Plant response to root knot nematodes has been recently reviewed by Bird (1974). The pathogenesis of nematode-infected plants has been discussed by Endo (1975).

The mechanical injury directly inflicted upon plants by the nematodes during feeding is slight. Most of the damage seems to be caused by a secretion injected into plants while the nematodes are feeding.

This secretion, called saliva, is produced in three glands from which it flows forward into the oesophagus and is ejected through the stylet. Some nematodes are such rapid feeders that in a matter of seconds they pierce a cell, inject saliva, withdraw the cell contents, and move on. Others feed more leisurely and may remain at the same juncture for hours or days.

The sedentary forms (Heterodera and Meloidogyne) remain attached to one point in the tissue throughout their lives. The males of the species may or may not penetrate the roots but the females invariably get established in or on the roots in a fixed position. In such species it is the female which is responsible for the destruction of the host.

Nematode saliva has various functions, depending on the habit of the organism. The saliva of the plant nematodes seems to aid the parasite to penetrate the cell walls and possibly to liquefy the cell contents, making them easier to ingest and assimilate. The saliva, being toxic, proves disastrous to the plant tissues and its effects may reach up to the leaves even if the nematode is present only in the roots.

As a result of the action of this saliva anyone of the following host responses may appear as symptoms of the disease:

(i) Cellular hypertrophy and hyperplasia,

(ii) Suppression of mitosis,

(iii) Cell necrosis, and

(iv) Stimulation of growth.

Dropkin (1969) has discussed the various cellular responses to nematode infection.

The feeding of soine nematodes produces only slight trauma in hot cells. When the nematode Tylenchorhynchus dubius feeds upon the root hairs of Lolium perenne a spherical mass is formed at the tip of the stylet within the host cells.

For about 30 seconds the nematode remains quiet, after which its bulb begins to pulsate and the spherical mass starts diminishing in size and finally disappears after one minute.

In some associations, the stylet acts simply as the organ of penetration and suction tube. The nematode Paraphlenchus acontiodes penetrates the host cells of Pyrenochaeta terrestris and withdraws their contents within 2 to 3 seconds.

In certain other associations, such as Ditylenchus myceliophagus feeding upon Botrytis cinerea, there is delayed removal of contents. Progressive penetration is shown by the nematode Rotylenchus uniformis when feeding on the root hairs and cortical parenchyma cells of Bolium perenne as well as by the nematode Pratylenchus crenatus when piercing the epidermal cells of Poa annua rootlets.

Many plant parasitic nematodes affect only those cells upon which they feed or a limited number of cells in the immediate vicinity of the feeding site cells of infected tissues separate undergo hypertrophy and lose chloroplasts.

In certain host parasite combinations the pathogen stimulates changes in host cells, bringing about alterations in metabolism vital to the growth and development of the pathogen but do not bring about any cell destruction.

The nematode Tylenchulus semipenetrans brings about an increase in the size of the nuclei and nucleoli, the cytoplasm of the host cells becomes dense, their wan thickens and the vacuoles of the infected cells disappear.

Anguina and Mlltmguina species stimulate gall formation in leaves and flower parts of grasses and other plants. The gads develop here due to hypertrophy and hyperplasia of parenchymatous tissue with a central cavity harbouring the nematode. No syncytia develop here.

Members of the genera belonging to the family Heteroderidae, show the greatest morphological adaptation to parasitism. At the site of the feeding of the nematode, a group of cells develop into characteristic syncytia around the head of the parasite.

The cytoplasm becomes dense and the size of the nuclei and nucleoli elilarges considerably. The cell walls are usually altered. Species of Meloidogyne induce hyperplasia in the pericycle which results in the formation of galls.

Heterodera species do not induce extensive hyperplasia. Nacobbus batatiformis forms galls on the roots of sugar beets and some other plants. Here galls are formed as a result of hypertrophy of cortex and epidermal cells.

The stele is not affected. Syncytial mass is formed by the alteration of cens immediately adjacent to the nematode's head and extends longitudinally along the root.

In Heterodera infection syncytium is formed as a result of the enlargement of nuclei, incomplete dissolution of cell walls, disintegration of nucleoli, thickening of cell walls and by the cytoplasm becoming dense and granular.

Giant cells of Meloidogyne are formed as a result of the enlargement of nuclei and the cells become polyploid and undergo synchronous mitosis. The cytoplasm of these cells becomes granular and new cells are incorporated by cell wall dissolution. The walls of the syncytium become thickened and the cells of pericycle divide repeatedly.

The role of certain nematodes as vectors of plant viruses has been studied in recent years and there is an increasing awareness that they may also be involved in the transport and inoculation of other pathogens, notably bacteria and fungi attacking roots or other plant organs in the soil.

Several types of nematode-pathogen interaction are suggested by Pitcher (1965), of which the following are examples:

The namatodes transport the pathogen from the soil to the plant or from one part of the plant to another, and the pathogen may fail to develop fully in the absence of the eelworm, as in twist of grasses and cereals caused by the fungus Dilophospora alopecuri in association with Anguina tritici.

Much more important and more common are the interrelationships between nemaflities and viruses. Several plant viruses, such as grapevine fan-leaf virus, arabis mosaic virus, tomato, ring spot virus, tobacco black fing virus, raspberry ring-spot virus and tobacco rattle virus are transmitted by nematodes. Sixteen species of nematodes have so far been thought to be vectors of plant viruses.

The first demonstration that eelworms are capable of transmitting plant viruses was made by Hewitt, et al. (1958). They showed that the fan-leaf disease of grape-vines was spread by the eel worm Xiphinema index.

Several nematode fungus disease complexes are known. Fusarium wilt of several plants increases in incidence and severity when the plants are also infected by nematodes. Similar effects have also been noted in disease complexes involving nematodes ad Verticillium wilt, pythium damping off, Rhizoctonia and Phytophthora root rots.

Relatively few cases of nematode-bacterial disease complexes are known. In most of these the role of the nematode seems to be that of providing the bacteria with an infection court and to assist bacterial infection by wounding the host.

In the yellow ear rot of wheat the role of the nematode is to mechanically carry the bacterium Corynebacterium Iritici inside the ovaries of the wheat flowers. In strawberry infection by the leaf nematode and the bacterium Corynebacterium fascians, a more involved interaction between the two pathogens seems to exist. Plants inoculated with both pathogens produce the so called cauliflower symptom, quite distinct from either of the symptoms produced by each of the pathogens.

Almost all plant pathogenic nematodes lead part of their lives in the soil. Many of these live freely in the soil, feeding superficially on roots and underground stems, and although they may cause injury to plants, they are not strictly parasitic.

Even in the most highly specialized sedentary parasites, the eggs, the pre-parasitic larval stages, and the males are found in the soil for all or part of their lives. Soil temperature, structure (porosity), moisture and aeration affect the survival and movements of nematodes in the soil. Nematodes occur in greatest abundance at a depth of 0-1.5 cm in the soil. The distribution of nematodes in cultivated soils is irregular and is greatest in or around roots of susceptible plants.

The greatest concentration of nematodes is in the region of host roots. This is primarily due to the greater amount of food available and also to the attraction of nematodes to the substances released into the rhizosphere.

To these must be added the so called hatching factor the effect of substances exuded from the root which diffuse into, or are carried to the surrounding soil and markedly stimulate the hatching of eggs of certain species, although most nematode eggs hatch freely in water in the absence of any special stimulus.

Nematodes spread through the soil very slowly and by their own power. The speed of movement in the soil is dependent on pore diameter, particle size, water content of the soil and the diameter and relative activity of nematodes.

In addition to their own movement, however, nematodes can be spread by anything that moves and can carry particles of soil. Farm equipment, irrigation, flood or drainage water, animals and dust storms spread nematodes.

Temperature has a profound effect on the survival and multiplication of nematodes in the soil and also on their parasitism. Some of the soil factors affecting root knot nematodes were reviewed by Kincaid (1946).

Singh (1964, 1965a, 1965b, 1967) has reviewed the literature on the effect of the soil, temperature, moisture, aeration, soil texture, organic matter, rhizosphere, and various cultural operations on the activity of plant parasitic nematodes.

Meloidogyne spp. have been reported to develop most rapidly at about 27 °C. Reproduction in M. halpa is extremely reduced at 35 °C. Ahmedand Khan (1964) found that the optimum temperature for the hatching of larvae in M. Intiognita is 30 °C. The temperature also affects the resistance of resistant varieties and the size of the gans formed by root knot nematodes on the host.

Soil moisture is important for nematodes in many ways. According to Parris (1948), Meloidogyne spp. can enter potato tubers through lenticels which occur on tubers commonly in wet soils.

Galling of tubers is less in dry soils. Other factors which influence the activity of plant parasitic nematodes are soil pH, soil texture, aeration, types of plants grown between the susceptible hosts, duration of cultivation of resistant varieties in the field, type of decomposable organic matter added to the soil, inorganic fertilizers, type of nitrogen and potassium status in the soil.

The phytopathogenic nematodes and their relatives comprise a large number of species placed in 15 families and 111 genera. The gross morphology of these nematodes, with few exceptions, is generally very similar. The body is covered by a cuticle. Internally there are digestive, excretory and reproductive systems.

The nematode body is more or less transparent. It is covered by a colourless cuticle which is usually marked by striations, or may show bristles, punctuations, warts, or other markings.

The cuticle molts when the nematodes go through their successive larval stages. The cuticle is produced by the hypodermis which consists of living cells and extends into the body cavity as four chords separating four bands of longitudinal muscles. These muscles enable the nematode to move. Additional specialized muscles exist at the mouth and along the digestive tract and the reproductive structures.

The oesophagus (pharynx) which is the anterior end of the digestive system consists in the Tylenchida, of stylet (stomatostyle), corpus, isthmus and glandplar posterior enlargement. Modifications in the morphology of the oesophagus are widely used at all levels of nematode taxonomy.

Many of the groupings of higher taxa in the Tylenchida, as well as in those groups of Dorylaimida containing phytoparasitic nematodes, are based largely upon modifications of gross shape and internal morphology of the oesophagus. The digestive system is a hollow tube extending from the mouth through the buccal cavity, oesophagus, intestine, rectum and anus. Lips, usually six numbers, surround the mouth.

All plant parasitic nematodes have a protrusible hollow stylet or spear. This is the main characteristic that suggests that the nematode is a plant parasite. This structure has been homologized in Tylenchida with the stoma of other nematodes and is used as a piercing organ through which nematodes acquire food. It is actually a three-part structure taxonomically; the stylet is used at all levels of classification.

The body cavity is rudimentary in its development and contains a fluid through which circulation and respiration take place.

The excretory system is not well developed in nematodes. However, the nervous system is well developed and consists of many nerves, ganglia and sensory structures.

The reproductive systems are well developed. The female nematodes have one or two ovaries followed by an oviduct and uterus terminating in a slit-like vulva.

The male's reproductive structure is similar to the females but has a testis, seminal vesicle and ejaculatory duct, and terminates in a common cloaca with the intestine. A pair of protrusible, copulatory spicules are also present in the male. Reproduction in the nematodes is through the eggs and may be sexual, hermaphrodite or parthenogenetic. Many species lack males.

The life histories of most plant parasitic nematodes are, in general, quite similar. There are six stages in the life cycle of a nematoded Egg, L ₁ (larval stage), L ₂, L ₃, L 4 and Adult. Eggs hatch into larvae, whose appearance and structure are usually similar to those of the adult nematodes except for the development of the reproductive systems.

Larvae grow in size and each larval stage is terminated by a molt. After the final molt the nematodes differentiate into adult males and females. The females can then produce fertile eggs either after mating with a male or in the absence of males, parthenogenetically, or they can produce sperm themselves.

The time for the completion of the life cycle s about 20 -40 days under optimum environmental conditions but longer in cooler temperatures.

In some species of nematodes the first or second larval stages cannot infect plants and they depend for their metabolic functions on the energy stored in the egg. When the infective stages are produced, however, they must feed on a susceptible host or else starve to death. If suitable hosts are not available, all the individuals of certain nematode species may die but in other species the eggs may remain dormant in the soil for years.

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Nematodes: Symptoms, Injury to Plants, Characteristics and Life Cycle| Parasites

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ADVERTISEMENTS: In this article we will discuss about:- 1. Introduction to Nematodes 2. Symptoms Caused by Nematodes 3. Mechanism of Nematode Injury to Plants 4. Interrelationship between Nematodes and Other Plant Pathogens 5. Ecology and Spread 6. Characteristics 7. Life Cycle. Contents: Introduction to Nematodes Symptoms Caused by Nematodes Mechanism of Nematode Injury to Plants ...

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Polyhouse Cucumber Nematode Disease and Solution

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Polyhouse Cucumber Nematode Disease and Solution

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Polyhouse Cucumber Nematode Disease and Solution | Summer update | August Update

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Agricultural soils generally support less than 100 nematodes in each teaspoon (dry gram) of soil. Grasslands may contain 50 to 500 nematodes, and forest soils generally hold several hundred per teaspoon. The proportion of bacterial-feeding and fungal-feeding nematodes is related to the amount of bacteria and fungi in the soil.

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Nematodes are microscopic worms that can live in your soil or water. Some nematodes are beneficial, controlling grubs and other insects in the soil. Other nematodes are harmful to plants, burrowing into roots and making their way up the stems and leaves. The damage makes it difficult for plants to absorb nutrients, ...

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Plant disease - Nematode diseases: Root-knot nematodes (Meloidogyne species) are well known because of the conspicuous "knots," or gall-like swellings, they induce on roots. More than 2,000 kinds of higher plants are subject to their attack. Losses are often heavy, especially in warm regions with long growing seasons.