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(See the WSU Hort Sense page on Gray Mold)
Gray mold rot (Botrytis blight) is a common disease on plants, especially nonwoody plants, grown under humid conditions. The fungus which causes the disease, Botrytis cinerea, attacks more than 80 kinds of plants in the Pacific Northwest. It is, however, serious on only a few of them.
Gray mold can be a problem in greenhouses when humidity is high and temperatures are moderate, attacking tomatoes and, occasionally, cucumbers. It is also a problem in the field, especially in western Washington, where it attacks green beans and the fruits of strawberries, raspberries, and blueberries. In eastern Washington, it occasionally causes a leaf blight on onions. It is sometimes a problem on ornamentals-particularly African Violets and other succulents.
Botrytis blight affects plants in a number of ways. It may cause collapse and damping-off of seedlings, blossom blight, fruit rot, stem and crown rot, or shoot blight. The first symptom is usually a water-soaked spot. The tissue later becomes soft and watery. The affected parts of the plant wilt and collapse. If the humidity remains high, a grayish-brown coating or web of mycelium (fungus threads) and spores develops over the surface of the collapsed tissue.
Many spores are produced which are easily blown or splashed onto healthy foliage. If a film of moisture is present and other conditions are favorable, germination and infection can take place in a few hours.
Sclerotia may be produced on fleshy parts of stems and fruits. These are durable resting structures that permit the fungus to survive when conditions are not favorable for growth. They vary in size up to 1/4 inch and are flattened and black. Sclerotia are not always conspicuous, but may be embedded in decayed tissue or coated with soil and other debris.
A film of moisture is necessary for spores to germinate and infect plants. Therefore, gray mold is favored by humid and relatively cool conditions. Opening up plantings by cultivation, by wider spacing of rows and plants, or by pruning permits freer movement of air and helps reduce humidity. In greenhouses, good ventilation will help to control gray mold.
The fungus thrives on plant debris.
Sanitation is an essential part of gray mold control. Fallen leaves and dead plants should be removed from greenhouses and burned, buried, or placed where they will dry rapidly. This will greatly help reduce the amount of infectious material in the area.
When gray mold is severe, as it may be on strawberries and raspberries in western Washington, or on tomatoes or other greenhouse crops, fungicidal sprays may be needed to protect healthy plants. A number of fungicides are effective. These include Botran, Benlate, captan, ferbam, Bravo, Daconil 2787, Chipco 26019, Ronilan, thiram, and fixed coppers. Because of the different rates and restrictions for different crops, consult the label for instructions before applying fungicides. Do not apply any of these materials to plants not listed on the label. (Tradenames have been used to simplify information; no endorsement in intended.)
For more information contact your local WSU Extension office.
