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How are pathogens spread and controlled | Health | Biology | FuseSchool

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Airborne spread of plant disease - Lagrangian stochastic model

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more detailed notes on plant transmission

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more detailed notes on plant transmission

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Plant disease - Transmission: With the exception of tobacco mosaic virus, relatively few viruses or viroids are spread extensively in the field by contact between diseased and healthy leaves. All viruses that spread within their host tissues (systemically) can be transmitted by grafting branches or buds from diseased plants on healthy plants.

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What do you notice about the leaves of the wheat on the right? How about on the left?

See those yellow and red spots on the leaves on the left? Those wheat plants are sick. They have a disease called wheat leaf rust. Wheat leaf rust is caused by a fungus, and like many fungi, it spreads from place to place in tiny particles called spores. When wheat rust spores land on the leaves of a healthy wheat plant, that plant can become infected. According to the Cereal Disease Lab , wheat rust absorbs nutrients from the plant tissues and makes that plant more susceptible to infection by other fungi and bacteria.

Wheat leaf rust ( Puccinia graminis ), can become a serious problem for a farmer if left untreated, but it isn't the only plant pathogen that can cause crop damage. There are thousands of species of fungus or fungus-like organisms that cause diseases in plants and animals. Most pathogens in this group are specialists, meaning they infect only one or a few different species, but fungi are the leading cause of crop loss worldwide , and the problem may become worse with longer periods of warmth and moisture due to climate change . Chemicals called fungicides can be used to kill disease-causing fungi, but they can be expensive and sometimes dangerous to people.

Part of tackling the problem of plant disease is to figure out how these infections move from plant to plant. In the following activity, you will model disease transmission between plants by rain, then devise a way to reduce the spread of a pathogen like the one that causes wheat leaf rust.

How Do Diseases Move From Plant To Plant?

Many human diseases are transmitted through violent expiration events , like coughs or sneezes. These clouds of droplets can carry pathogens and even suspend them in air. But plants don't sneeze, so how are diseases spread from plant to plant in a field or greenhouse?

To answer this question, let's start by thinking about the abiotic (nonliving) and biotic (living) factors present in the ecosystem that the plant belongs to, even if that ecosystem exists inside a greenhouse or on a farm. What could move a fungal spore from one plant to another? How could abiotic factors influence the transmission of a (biotic) fungal pathogen like wheat rust?

Related Video

Breakthrough: Connecting The Drops

Diseases that attack the leaves of a plant are primarily spread by wind, but they can also move to nearby plants by taking a ride on splashing water droplets from rain or irrigation. How can we change how crops are planted and grown to minimize the spread of pathogens by water droplets?

This is one of the questions that Dr. Lydia Bouriba, a scientist studying fluid dynamics at the Massachusetts Institute of Technology, attempts to answer with her research. Her lab studies how small droplets can transmit plant and human diseases.

Raindrop Range Of Contamination.

How far can fungal spores travel in water droplets that splash off an infected plant? In the following investigation, you will drop water onto the leaf of a plant and analyze the size and distribution of the droplets that splash off the leaf. Based on your data, you will make a recommendation to a farmer preparing to plant tomatoes in a greenhouse.
Note: For the purposes of this investigation, we are only addressing disease that can be transmitted by droplets.
Plant Observations.

Before starting your splatter investigation, examine the leaves of your plant and make a list of observations. Observe things like how much each leaf bends, the size and texture of the leaves, and any other physical traits. Which of those traits that you observed might affect how far a droplet will bounce off the surface?

Materials (Per Group Of 2-4 Students).

Small (3mL) plastic pipette or eyedropper

Ruler

Rubber band

3 medium binder clips

1 box (a printer paper box works well)

25 mL water, tinted with food coloring

Small potted plant
The plant should be upright, but does not have to be one of the crops we've discussed. We recommend a common ivy houseplant, such as Hedera helix , or a philodendron.

Polar graph paper
Create your graph paper by drawing concentric circles with diameters at 5 cm, 10 cm, 15 cm, 20 cm, and 25 cm using this technique . Draw an X in the center and label "Leaf tip."

2 colored pencils (different colors)

Optional: Yard stick to help center your leaf under the pipette or eyedropper

Optional: Drop cloth to protect floor

Optional: Camera or phone with video recording set to burst or slow motion

Conduct The Experiment.

1. Release one tinted water droplet.

2. If you are using a camera, take a photo of the splatter by placing your camera within the camera positioning marks.

3. Make a note of any drops that seem to combine to form "super splatters," pools of more than one drop.

4. Repe at for five water droplets.

5. Carefully remove your plant and move your polar graph paper so it can dry. You can carefully blot it with a tissue to speed up the drying process.

While Paper Is Drying, Discuss Your Observations.

Where are the largest (>5mm) splatter droplets located? Why do you think they landed where they did?

Where are the smallest splatter droplets located? Develop a hypothesis for why droplets might travel different distances based on whether they are large or small.

Did any of the droplets splatter beyond your graph paper? Were they large droplets or small droplets?

Related Segment

Back-To-School Lessons From Science Friday's Educator Collaborative

Collect Splatter Data.

1. Count the large splatter droplets that fell in each circle band. Write the total for each inside the respective circle band. As you count, cross out the droplet with a small "x" so you only count it once.

2. Count the small splatter droplets. Using a different color, write the total inside the circle band in a different color. Again, cross out the small droplets with a small "x" as you count.

3. Repeat, counting and recording the small and large droplets within each band.

4. Create a histogram that shows the range of large and small drops from your polar graph paper.

Analysis Questions.

How might droplet size affect how fungal spores are passed to other plants?

Describe the pattern of droplets. How far away from your plant would you expect spores to spread?

What was your maximum droplet splatter range? Examine the other leaves on your plant. Do you think you would get the same result for each leaf? What might determine how far droplets spread for any particular leaf?

Make A Recommendation.

Based on the information from your plant splatter experiment, make a recommendation for changes to plant spacing or the irrigation system in a greenhouse growing that plant. In your recommendation, please be sure to refer to your splatter test data and explain your rationale for your recommended changes based on that data. Take the following into consideration:

In a field or greenhouse, you are dealing with the splatter of many plants in many directions.

Farmers must be able to make a profit off their crops, and try to grow as many plants as possible while losing as few plants as possible to disease. Try to space plants to minimize disease, while maximizing the amount of plants that can be grown in a given area.

Are there other changes you might recommend to this farmer? Be sure to explain how they will reduce disease transmission.

Reflection Questions.

The droplet splatter setup in this experiment is a model that helps us explore the interaction between one droplet and one leaf. What are the limits of this model? What parts of real rainfall does it fail to represent?

Redesign this droplet splatter investigation or design a new investigation that would address some of the limitations you identified in the previous question.

Extensions.

Develop a formula that optimizes output (crops per square meter), but minimizes disease transmission. Be sure to take the growth of the plant (size) and the maximum range of contamination. What information would you need in order to be able to apply your formula before you planted any crops? How would your formula need to change in order to be applicable to different types of plants?

Repeat the experiment above, but using a crop grown in your region. Try to find a model plant that best mimics the leaf properties of that crop if you cannot use a real one. Interview local farmers about techniques, aside from fungicide, that they use to control the spread of fungus in their crops. Based on your interview with farmers and experimental data, write a recommendation for plant placement.

How can we change how crops are watered to lessen diseases that are spread by droplets? Design an experiment to test how different watering methods might increase or decrease spread of disease by droplets.

Related Resources.

Breakthrough: Connecting the Drops Bacteria and viruses hitch a ride inside droplets of all kinds-sneezes, raindrops, toilet splatter. By reviewing footage of different types of drops, applied mathematician Lydia Bourouiba records and measures where they disperse in order to better understand how diseases spread. Watch how Bourouiba designs tests-some inescapably humorous and awkward-to study infectious disease transmission.

Raindrops splash down on leaves, spread pathogens among plants. Dr. Bouriba describes the work of a team of researchers from MIT as well as the University of Liege, in Belgium, that uses high-speed images of raindrops splashing down on leaves to show that raindrops can act as a dispersing agent of contaminated droplets from one plant to another.

Next Generation Science Standards.

Credits.

Resource writing and photography: Beth Topinka, Xochitl Garcia

Illustrated diagrams: Ari Tiziani

Editing: Ariel Zych, Laura Helft, Johanna Mayer

Digital Production: Johanna Mayer

If you have questions about this resource or would like to share your experience using it with students, please e-mail us at educate@sciencefriday.com or tweet to @scifri

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Plants Don't Sneeze, But They Can Still Catch A Cold

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What do you notice about the leaves of the wheat on the right? How about on the left? See those yellow and red spots on the leaves on the left? Those wheat plants are sick. They have a disease called wheat leaf rust.

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Infectious diseases are transmitted from person to person by direct or indirect contact. Certain types of viruses, bacteria, parasites, and fungi can all cause infectious disease. Malaria, measles, and respiratory illnesses are examples of infectious diseases.

Simple preventative measures, such as frequent hand washing, can cut down on disease transmission.

Infectious diseases are often spread through direct contact. Types of direct contact include:

1. Person-to-person contact

Infectious diseases are commonly transmitted through direct person-to-person contact. Transmission occurs when an infected person touches or exchanges body fluids with someone else. This can happen before an infected person is aware of the illness. Sexually transmitted diseases (STDs) can be transmitted this way.

Pregnant women can also transmit infectious diseases to their unborn children via the placenta. Some STDs, including gonorrhea, can be passed from mother to baby during childbirth.

2. Droplet spread

The spray of droplets during coughing and sneezing can spread an infectious disease. You can even infect another person through droplets created when you speak. Since droplets fall to the ground within a few feet, this type of transmission requires close proximity.

Infectious diseases can also be spread indirectly through the air and other mechanisms. For example:

1. Airborne transmission

Some infectious agents can travel long distances and remain suspended in the air for an extended period of time. You can catch a disease like measles by entering a room after someone with measles has departed.

2. Contaminated objects

Some organisms can live on objects for a short time. If you touch an object, such as a doorknob, soon after an infected person, you might be exposed to infection. Transmission occurs when you touch your mouth, nose, or eyes before thoroughly washing your hands.

Germs can also be spread through contaminated blood products and medical supplies.

3. Food and drinking water

Infectious diseases can be transmitted via contaminated food and water. E. coli is often transmitted through improperly handled produce or undercooked meat. Improperly canned foods can create an environment ripe for Clostridium botulinum, which can lead to botulism.

4. Animal-to-person contact

Some infectious diseases can be transmitted from an animal to a person. This can happen when an infected animal bites or scratches you or when you handle animal waste. The Toxoplasma gondii parasite can be found in cat feces. Pregnant women and people with compromised immune systems should take extra care (disposable gloves and good hand washing) when changing cat litter, or avoid it altogether.

5. Animal reservoirs

Animal-to-animal disease transmission can sometimes transfer to humans. Zoonosis occurs when diseases are transferred from animals to people. Zoonotic diseases include:

6. Insect bites (vector-borne disease)

Some zoonotic infectious agents are transmitted by insects, especially those that suck blood. These include mosquitos, fleas, and ticks. The insects become infected when they feed on infected hosts, such as birds, animals, and humans. The disease is then transmitted when the insect bites a new host. Malaria, West Nile virus, and Lyme disease are all spread this way.

7. Environmental reservoirs

Soil, water, and vegetation containing infectious organisms can also be transferred to people. Hookworm, for example, is transmitted through contaminated soil. Legionnaires' disease is an example of a disease that can be spread by water that supplies cooling towers and evaporative condensers.

Because infectious diseases can spread through direct or indirect contact, everyone is at risk of illness. You have a higher risk of becoming ill when you're around sick people or in areas susceptible to germs. If you work in or visit a care center, a day-care center, a hospital, or a doctor's office, take extra precautions to protect yourself.

1. Illness

Something as simple as touching a doorknob, elevator button, light switch, or another person's hand increases the likelihood of coming in contact with germs that can make you sick. The good news is that a few simple precautions can prevent some disease transmission. For example, make sure you wash your hands frequently and thoroughly. Use soap and warm water and vigorously rub your hands together for at least 20 seconds. If you can't wash your hands, use an alcohol-based hand sanitizer. Washing your hands is the gold standard though!

Other tips to prevent the spread of disease in areas with germs include:

wash your hands or use hand sanitizer before handling food and after shaking hands

always wash with soap and water if your hands are visibly soiled

try to minimize touching your mouth or nose with your hands

avoid sick people, if possible

wear disposable gloves to avoid contact with blood and feces

use disposable gloves when caring for an ill person

cover your mouth when you sneeze and cough and wash your hands afterward

teach children not to put their hands or objects in their mouths

sanitize toys and changing tables

2. Foodborne illness

Dangerous organisms can thrive in improperly prepared food. Avoid cross-contamination by keeping raw meats and produce separate. Use different preparation surfaces for raw meats and wash surfaces and utensils thoroughly.

Freeze or refrigerate perishable foods and leftovers promptly. According to the United States Department of Agriculture, you should set your refrigerator to 40°F (4°C) or below and your freezer to 0°F (-18°C) or below. Cook meats to a minimum internal temperature of 145°F (63°C). Cook ground meats to 160°F (71°C) and poultry to 165°F (73°C).

Be careful about sources of food when visiting foreign countries.

3. Insects and animals

When camping or enjoying wooded areas, wear long pants and long sleeves. Use insect repellent and mosquito netting. Don't touch animals in the wild. Don't touch sick or dead animals.

4. Vaccinations

Stay up to date on vaccinations, especially when traveling. Don't forget to keep your pet's vaccinations current, too.

Vaccinations can drastically reduce your risk of becoming ill with some infectious diseases. If you can avoid a particular disease, you can also prevent the spread of the disease. There are different types of vaccinations, such as those to prevent:

Speak with your doctor to discuss the benefits and risks of these and other vaccinations.

Infectious diseases are caused by types of bacteria, viruses, parasites, and fungi around us. It's important to understand how these diseases are transmitted. If you understand the transmission process, you can use this knowledge to protect yourself and help prevent the spread of illnesses.

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how are diseases transmitted? diseases transmitted through indirect contact

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disease transmitted through direct contact

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Disease Transmission: Direct Contact vs. Indirect Contact

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Infectious diseases are transmitted from person to person by direct or indirect contact. Certain types of viruses, bacteria, parasites, and fungi can all cause infectious disease. Malaria, measles, and respiratory illnesses are examples of infectious diseases. Simple preventative measures, such as frequent hand washing, can cut down on disease transmission.