Sclerotinia sclerotiorum has a very wide host range, attacking more than 300 plant species. It becomes a problem on tomatoes when cool and wet weather occurs during crop development.
Symptoms of Sclerotinia sclerotiorum reflect its common name, White Mold, and consist of prominent white, cottony mycelium covering affected plant parts. Survival structures, known as sclerotia, which are about the size of mustard seeds, black in color, and resemble mouse droppings, develop on diseased tissue and within killed stems. Infected stems have a bleached, light gray, dessicated appearance. Initial lesions are small, circular, water-soaked and light green but rapidly increase in size. Affected tissues dry, turn brown, and may be covered with a white, cottony mycelium. Sclerotia form in infected tissue and entire branches or plants may be killed. Typically, infected fruit rapidly disintegrates with a watery rot and ring of sclerotia develops around the calyx.
Sclerotinia sclerotiorum overwinters in the soil as sclerotia (small, black resting structures) and can persist there for 5-8 years. White mold develops after or during the flowering period, as the fungus needs the senescing tissue for nutrition to begin the infection process. Infection usually begins in leaf axils or stem joints where fallen petals have lodged. The stems are invaded, become soft, and eventually die. The pathogen may also enter the plant at the soil line and through fruit. The fungus is favored by cool, moist weather, high humidity, and long periods of leaf wetness.
Cultural Controls & Prevention:
- Deep plowing and crop rotation are of limited value because of the wide host range of the pathogen and its ability to persist in the soil for extended periods. Sclerotia buried by deep plowing may subsequently emerge at soil surface with later cultivation.
- White mold is more severe where the plant canopy is dense; reduction in canopy density can be achieved by increases in row width, plant spacing, cultivar selection, and careful attention to nitrogen levels.
- Reduce humidity and high moisture periods within the field by orienting rows in the direction of prevailing winds, avoiding excessive irrigation after petal fall, and timing irrigation to allow plants to dry before nightfall.
- Rotate with non-hosts for up to 8 years. Non-hosts include grasses, cereals, and onions.
- Losses of fruit can be minimized by timely harvesting, rapid cooling, and storage under refrigeration.
A biological agent, Coniothryium minitans (Contans) can reduce field populations of S.sclerotiorum by parasitizing sclerotia. Contans must be incorporated into the soil and is best applied 3-4 months before crop planting or in the fall.
Chemical Controls & Pesticides:
For Current information on disease recommendations ins specific crops including information on chemical control & pesticide management, please visit the New England Vegetable Management Guide website.
Crops that are affected by this disease:
- Beans, Snap, Dry, and Lima
- Beet and Swiss chard
- Cabbage, Broccoli, Cauliflower, and Other Brassica Crops
- Carrot and Parsnip
- Cucumber, Muskmelon, and Watermelon
- Lettuce, Endive, Escarole
- Pumpkin, Squash, and Gourds
- Rutabaga and Turnip
- Tomato, Field
- Sweet Potato
- Tomato, Greenhouse