Plant Pathology Guidelines for Master Gardeners
Contact: Dr. Richard Raid
Plant Disease Control
It is very important to remember that a correct diagnosis is the most important step in the eventual control of a plant disease. Most diseases have a fairly well established control protocol. Most often, failure to control the disease happens because the problem was misdiagnosed in the first place.
This is a list of the most important general strategies for management of plant diseases:
- Crop Resistance - should be first line of defense whenever possible
- Cultural Methods
- Physical Methods
These methods will be discussed further with examples.
Cultural methods for disease control refer to those growing methods that reduce pathogen levels or reduce the rate of disease development. These include:
- Crop Rotation
- Host Eradication
- Improvement of Crop Environment
How are the tomatoes is in this photo being watered? How does this help to reduce disease?
Note the tall sugarcane planted between rows of much shorter pepper. The sugarcane serves as a windbreak to reduce windblown sand injury that might create wounds for entry of pepper pathogens. The cane also traps winged aphids that might harbor and transmit several important pepper viruses. This is another excellent example of a cultural control.
What is the obvious cultural control in this picture?
What control method is being conveyed in this picture?
Do these cabbage seedlings, bought at a suburban garden center look completely healthy?
The following terms are important in a discussion of crop resistance:
Immunity is the rule in the plant kingdom; most plants are immune to most pathogens. Therefore, one does not have to worry that the black spot on roses will appear next year on the garden tomatoes.
Resistance, strictly speaking, refers to the lower disease levels seen in some cultivars or varieties of a particular crop species compared to other susceptible cultivars. Always choose resistant varieties when available.
Tolerance refers to varieties or cultivars that appear to be just as heavily diseased as standard varieties, but which manage to be more productive or vigorous.
Check out these seed packets. Where is disease resistance mentioned?
Note this cucumber variety. How many diseases is it resistant to?
Create an environment unfavorable to the pathogen and favorable to the crop.
There are many theories and ideas on the best time of the day to water. From a plant pathology point of view, it is preferable to water late morning, after the dew dries from leaves, but early enough to allow leaves to dry before evening. Of course, ideally, you would water only the soil surface because the roots are the water-absorbing organs.
Drip irrigation is used here. The water moves out into the field in the blue pipes which, in turn, feed water to small emitter lines that deliver water under the plastic mulch right at the base of plants.
Because the foliage doesn't get wet, development and spread of fungi and bacteria are much reduced.
Mulching is the obvious answer. Of course, there are many benefits to be gained from mulching, including weed control, soil moisture optimization, and soil temperature moderation. But mulches can serve as a barrier between above-ground plant parts and pathogens in the soil. Also, by reducing weeds and alternate hosts for pathogens, such as several viruses, mulches help in the battle against diseases.
This photo shows pruning shears being disinfected (a better word is disinfested) by immersion in a chemical. A good disinfectant is 10% Clorox or bleach (10 ml household bleach to 90 ml water). Isopropyl alcohol from the drugstore also works well. When pruning out diseased plant material, it is best to disinfest after each pruning operation. Also, make cuts several inches beyond diseased tissue in healthy tissue to make sure you get all of the pathogens.
These seedlings have typical "v-shaped" lesions characteristic of black rot, a serious cabbage disease. These could easily be sold to unsuspecting customers (but not well-informed Master Gardeners). Be aware that diseased planting material is out there. Obviously, one of the best ways to manage diseases is to keep them out in the first place. Inspect all planting material and be a discerning buyer!
You can see rust resistant snapdragons, VFN (Verticillim wilt, Fusarium wilt, nematode - resistant) tomato and mildew-resistant (powdery mildew) zinnia.
Four is correct: downy mildew, powdery mildew, anthracnose, and angular leaf spot.