White mold of beans is caused by Sclerotinia sclerotiorum, which also causes disease on more than 360 different plant species. It is a major disease of beans world wide, particularly in cool, moist regions and can result in complete crop loss.
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. 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.
White mold of beans develops after or during the flowering period, as the fungus needs the senescing tissue for nutrition to begin the infection process. The fungus is favored by cool, moist weather, high humidity, and long periods of leaf wetness. All aerial parts of the plants including pods and seeds may be attacked. Sclerotinia sclerotiorum overwinters in the soil as sclerotia (small, black resting structures) and can persist there for 5-8 years.
Severe losses of stand can occur when the pathogen attacks the stem and entire plants are killed. Wilting due to infection reduces plant vigor and yield. Pod infections reduce marketability and yield.
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 fresh pods 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:
Fungicide applications are of limited value as the pathogen is soil-borne and persists in the soil indefenitely.
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
- Tomato, Greenhouse
- Sweet Potato