White mold first appears as a watery rot on bean stems, leaves, and pods. Under moist conditions, white mycelium is often visible on the surface of rotted tissue. The development of black, irregularly shaped sclerotia is the best diagnostic feature. The disease may occur on the stem near the soil line or on pods, leaves, and stems. Affected tissue may appear bleached.
Life cycle
White mold develops most rapidly at 68 to 77°F and is generally more severe in gardens with heavy canopies. The black, irregularly shaped sclerotia survive in the soil for several years. After several weeks of moist and cool conditions, sclerotia within 2 inches of the soil surface are preconditioned to produce mushroomlike fruiting structures that form after one to several weeks of temperatures at 52 to 68°F in moist soil. Airborne spores are then released and germinate on plant parts, especially senescing flower parts. The fungus may remain viable in blossoms for a month. Plant surfaces in contact with blossoms must remain continuously wet for 48 to 72 hours for infection to occur.
Solutions
White mold is favored by a wet soil surface. Use of raised beds and careful furrow irrigation that does not overflow onto bed surfaces can help limit damage. Space plants well enough to allow good air circulation. Remove and destroy entire infected plants and crop residues as soon as you see them. Two years rotation out of susceptible crops (legumes, lettuce, cole crops, cucurbits) would be required for control of heavy soil infestations. Resistant varieties may be available.
