1. Plant Disease 101 Shannon Carmody Introduction
Ask: Tell me your name and your name and a memorable plant disease you’ve encountered.
Ask: What kind of things cause plant problems?
Plant Disease 101
Shannon Carmody

2. What is Causing your Plant Problem?
Biotic: living or biological
Pests
Fungi
Bacteria
Nematodes
Viruses
Other: parasitic higher plants, mollicutes, and protozoa
Pests: small pests include insects, large pests could include deer, rats, a neighbor
Fungi: like pests, some are visible with the naked eye, however, many are not. Fungi is the biggest group of plant pathogens, it is very diverse, and there are many different life cycles of fungi.
Bacteria: Are very small rarely seen with the naked eye, less diverse in life cycle and morphology.
Viruses: Are even smaller, sub-microscopic, spread by vectors like thrips, whiteflies, aphids
Nematodes:
Other pests might be parasitic higher plants, mollicutes, and protozoa

3. What is Causing your Plant Problem?
Abiotic: non-living or physical
Too much or lack of water
Extreme temperatures, humidity, light, water
Nutritional toxicities or deficiencies
Chemical injury
Mechanical injury
The first two bullets: too much or not enough water, light, humidity, or temperatures can kill your plants, cause very specific problems, but they can also intensify or exacerbate other plant problems. We’ll discuss some of these examples later.
Too much or too little nutrients can cause problems whether it is a macronutrient like Nitrogen, Potassium or Phosphorus or whether it is a micronutrient such as boron or molybdenum.
Chemical injuries could be from herbicides, and not just conventional herbicides, some organic sprays could damage your plants.
Mechanical injury might happen from equipment, and provide an entry point for biotic organisms.

4. The Disease Triangle Disease
Susceptible Host
When these 3 elements occur, plant disease can develop. Reducing just one of these elements helps you manage disease or reduce the potential amount of disease.
Conducive environment
Disease
So biotic and abiotic factors are things we are looking for, but these factors don’t cause disease on their own. Think of disease as a triangle with three required components: a susceptible host, conducive environment, and virulent pathogen. When you think about managing disease, think about reducing the impact any one of these components has, or in other words make the sides of the triangle smaller. For example, if you are overhead irrigating a crop, and you have a pathogen in your field that likes lots of moisture, switching to drip irrigation will significantly decrease the conducive environment available to the pathogen. Now, if you find a resistant cultivar to grow in your field, you’ve decreased the susceptible host available to your pathogen. Now the potential for disease in your field, at least for that particular disease is considerably reduced.
Virulent Pathogen

5. What should I do when I have sick Plants?
Diagnosis
Use online resources, extension bulletins, compendia, and other resources:
PNW Vegetable Extension Group: path_team.htm
PNW Plant Disease Mgmt. Handbook:
American Phytopathological Society:
I am hoping for 2 outcomes in this workshop. This first is that when you see a plant problem in the field you might be able to diagnosis it yourself. There are boatloads of resources out there to help you. Here are a few, Dr. du Toit and a few other folks from WSU put together the PNW Vegetable Extension Group. I’ve used a lot of pictures from the site in this presentation, but basically you can go to this website and see what kind of pathogens are common in this area, find pictures, and get resources for how to manage them.

6. What should I do when I have sick Plants?
Submit a sample to a diagnostic lab
Private labs
WSU Puyallup Plant Clinic (Jenny Glass)
WSU Northwestern Washington Research and Extension Center
Entomology
Plant Disease
Weed Science
Horticulture
Livestock
Skagit County Extension
Other experts in the valley
My second outcome, is probably the more realistic one. It is likely that you will have a plant problem and not know what it is. In the case of many biotic problems you might run into, you might not have the necessary equipment or expertise to make a diagnosis. There are many plant clinics and experts in the state that can help you with a diagnosis. They’ll have microscopes, tissue culture labs where they can isolate the pathogen from your sample to figure out what they are. They may also extract and look at the DNA of pathogen in your samples. The point is there are lots of tools plant clinics have to figure out what’s wrong with the plant. So your best option might be submitting a sample, and taking a good sample is key to a quick and proper diagnosis.

7. Consider the disease triangle when taking samples
Provide basic crop information:
Age
Weather
Size
Soil
Cultivar
Cultural practices
Location
Pest management history
Exposure
To take a good plant sample, consider the factors from the disease triangle. In the hand outs I’ve included an example of a sample submission form, and it includes lots of the details you see on this page – age of the plant, size, cultivar, location of the plant and the field/farm, exposure, weather, type of soil you might have, any relevant cultural practices, and pest management history.
Are there any questions about this stuff?

8. Consider the disease triangle when taking samples
Ask these 5 questions:
What was the timing of your plant problem?
Is more than one plant species affected?
How is the field affected on a macro level?
How is the plant affected on a micro level?
What symptoms and/or signs can you observe?
In addition to that basic info think about these 5 questions, we will focus on these for the remainder of the lecture. And again, these questions really are just filling out the 3 sides of the disease triangle

9. What was the timing of your plant problem?
When did symptoms first appear?
How quickly did symptoms develop?
Are symptoms progressing or are they static or declining?
What has happened recently, or in the past, in this area?
When did symptoms first appear? Right after some event like spraying or planting? Did it develop in certain part of the season?
How quickly did symptoms develop? What could you guess about the problem if it developed over night?
Are symptoms progressing or are they static or declining? What could you guess about the plant problem if the symptoms spread vs. stayed the same? Think back to when we discussed those biotic and abiotic factors.
What has happened recently, or in the past, in this area? Did a neighboring farm harvest a crop? How could that affect you?

10. Is more than One Plant species Affected?
Fungal and bacterial pathogens tend to be host specific, meaning that they will only affect one species. Some abiotic factors may affect more than one species, which provides a good clue to the kinds of things you can start looking at for your plant problem. What are some examples of factors that could affect multiple species?
Is more than One Plant species Affected?
What does this tell you about your plant problem?

11. How is the Field affected on a macro Level?
First we’re going to look at the whole field. This is spinach, and it’s kind of hard to see but there are some dark patches where the spinach is not growing. Some other observations you might make about this field, is that it’s not very dark green and there are some lighter patches, so maybe there are some nutritional deficiencies happening here.

12. Field distribution of symptoms:
Disease in fields usually appear more random than this. If you see something that uniformly affects our field start thinking about abiotic factors.
Field distribution of symptoms:
Linear and repetitive

13. Photos by Dr. Lindsey du Toit
This isn’t always straight forward though. This is a field of cabbage being grown for hybrid seed. There two rows on the left are the male line, the two on the right are the female line. You can see that the male lines are in bad shape, not growing, yellowing of the lower leaves. But it is very uniform, so much so that you would not think that is a biotic problem. Any ideas on what is happening here? The male line is less cold tolerant and froze over the winter, which you can see here in this picture. Weather, extreme temperatures, are the abiotic factor at play.
Photos by Dr. Lindsey du Toit
mtvernon.wsu.edu/path_team/vegpath_team.htm

14. Field distribution of symptoms:
This sort of a pattern is usually indicative of certain insects, such as leafhoppers, or maybe a disease that is transmitted by seed or infected transplants.
Field distribution of symptoms:
Individual and random

15. Beet curly top virus (BCTV) Leafhopper vector
Here’s an example of a virus, beet curly top virus, transmitted by a leafhopper on a cilantro seed field in eastern Washington. Because viruses are not visible with the naked eye, what kind of clues would you look for in this field to give you a clue as to what’s happening?
Beet curly top virus (BCTV) Leafhopper vector
Photos by Dr. Lindsey du Toit
mtvernon.wsu.edu/path_team/ve gpath_team.htm

16. Field distribution of symptoms:
This sort of distribution could mean a number of things – maybe a pathogen from a neighboring field or insects with that can’t move as far as leafhoppers. It is important to note that this sort of distribution appears to be spreading.
Field distribution of symptoms:
Edge of field, diffuse

17. Iris yellow spot virus in onion bulb and seed crops
Photo by M. Trent
Iris yellow spot virus in onion bulb and seed crops
Vector = onion thrips
Here’s an example from E. WA of Iris Yellow Spot Virus on onions. This is transmitted by thrips.

18. Field distribution of symptoms:
This distribution also show’s signs of spreading, it could be nematodes, bacteria, or fungi.
Field distribution of symptoms:
Hot spots: diffuse or sharp borders

19. Stubby root nematode Photos by Gary Pelter
Nematodes are roundworms, and very abundant. Plant pathogenic nematodes feed off of the plant cells causing stunting which you can see here in this image of the field and the onion roots.