Gray Mold a Problem for Many Crops

gray-leaf-spotGray mold is a fairly common problem in tomato and can be a major cause of postharvest rot at harvest and in storage. In addition to tomato, gray mold can attack beans, cabbage, lettuce, muskmelon, pepper, and potato, as well as many ornamentals.

Identification
The gray mold fungus, Botrytis cinerea, derives its name from the Greek word botrys, meaning a bunch of grapes, which describes the characteristic arrangement of spores.

Botrytis can cause a variety of problems including damping-off and blights of flowers, fruits, stems, and foliage. Entry often occurs through damaged tissue. Stems can become infected through leaf scars, dead leaves, or other forms of stem damage. Stem lesions appear as large elliptical water-soaked lesions. These may partially girdle the stem, but sometimes the entire stem is affected and the plant is killed.

Leaf lesions develop into wedge-shaped grayish-brown lesions. During cool moist weather, a gray fungal growth may be evident on infected tissue.
Fruit often are infected at the stem end or shoulder where they contact other infected plant parts. Young fruit also can become infected through the air. Water-soaked spots appear with a light brown to tan central region. Decay progresses rapidly. A soft rot may develop with the fruit skin remaining intact, while the inner tissue becomes mushy and watery. Sclerotia may form in infected tissues.

If there is a rapid weather change (not favorable to the fungus), fruit infections may abort. White circular (halo) spots appear on the fruit and are called “ghost spots.” These spots persist on green, breaker, and mature fruit.

Survival And Spread
The fungus survives between crops as sclerotia or as mycelium in plant debris. Other crops also may serve as sources of inoculum. Development is favored by cool, wet, humid weather. Airborne spores landing on tomato plants germinate and can produce an infection when free water from rain, dew, fog, or irrigation is present for prolonged periods. Senescent flower parts that have fallen onto leaves are a common starting point for leaflet colonization. Leaf lesions often start on senescent tissue or areas of physical or chemical damage.

Management Methods
Some research indicates that disease development is favored by low calcium to phosphorus levels in the soil. There is no known resistance to B. cinerea in tomato cultivars. Scala SC (pyrimethanil, Bayer CropScience) and Switch 62.5 WG (cyprodinil and fludioxonil, Syngenta) are labeled for control of this disease. Other fungicides including chlorothalinil, applied for the control of other diseases may provide some protection.

By Gene McAvoy, Regional Vegetable Specialist IV, University of Florida/IFAS