||White mold is a sporadic disease which is most serious when wet weather occurs at flowering. When the surface soil is wet, the hard black survival structures, called sclerotia, germinate to form tiny mushroom-like bodies that liberate millions of wind-borne spores. These spores colonize dead bean tissue, particularly the dead blossoms, then the fungus invades green tissues, causing a watery soft rot. In wet weather infected tissues are tan colored and soft, with|
tufts of fluffy white fungal growth.
These tufts of "white mold" develop into hard black bodies, the sclerotia, some of which survive many years in the soil. Leaves of infected plants turn yellow and wilt. In dry weather affected stems have a bleached or whitish appearance. Infected seed is discolored, chalky and lightweight.
White mold is favored by rainy weather before and at flowering, moderate temperatures and long periods of high humidity, and keeping the lower canopy of plants wet more or less continuously for up to two days.
Crop rotation is important but of only modest value in areas of intensive bean production, since the spores may blow in from nearby infested fields. Benlate and Topsin M are registered for white mold control. They are more effective when applied before infection occurs. Early bloom is the best time to apply these fungicides. Good canopy penetration is required so that the blossoms and lower stems are covered with fungicide. The most economical method is band application using drop nozzles, high pressure and high gallonage. High pressure broadcast application is not quite as effective but can also be used when band application is impractical. Aerial application using 7½-10 gpa also can be effective. Widely spaced rows may help enhance drying in the canopy. Upright varieties dry more quickly and may escape severe infection in years with conditions that are marginal for white mold development. Deep plow infected bean crop refuse and clean harvest equipment between fields. Avoid short rotations or rotation with other susceptible crops, especially beans, sunflower, canola, lentils and soybeans.
SOURCE: NDSU Extension Service Circular A-1133