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White Mold
White mold (sometimes called watery soft rot) is a fungal disease (Sclerotinia sclerotiorum) which threatens bean production in North Dakota. White mold can cause substantial yield losses.
White mold is difficult to eliminate because the fungus forms tough black to grayish bodies called sclerotia which can survive more than 10 years in the soil. These rarely germinate to produce a white fungal mat which infects lower bean stems directly. More commonly they germinate to produce small fruiting structures 1/8 to 3/16 inch in diameter and shaped like funnels. As many as 40 fruiting structures can arise from a single sclerotium. The fruiting bodies release airborne spores that are dispersed throughout the growing season. The spores cannot infect healthy plant tissue directly, but germinate on date plant tissue (such as dried blossoms, broken leaves lodged in the foliage, etc.); the fungus then proceeds into healthy tissue.
Spread may appear to be rapid. Following a spore shower, an entire field can be lost only days after initial symptoms are detected. Debris from infected plants or plant contact can spread the pathogen.
Sclerotia and watery soft rot are the main diagnostic characteristics of the disease. Infected plants first develop small, water-soaked spots on the pods, stems or foliage. The spots enlarge to form large masses of soft-rotted tissue covered with masses of white moldy fungus growth (hence the name). In dry weather, infected plants appear yellowish brown, bleached, dried, and shriveled as if they had been cut from the roots. Late season infections make the bean seeds chalky-colored and lightweight.
The fungus may be brought into a field through infected bean seed, through flood and irrigation water, through sclerotia in seed lots (sunflower and bean), through windblown soil or by man.
White mold disease develops best in moderate temperatures (about 75 G, 24 C), but fungus fruiting bodies develop best at cool temperatures (about 60 F, 15 C). Dew, rain, and irrigation can provide moisture required for disease progress.
Vining beans, shelterbelts, windrowing and other field conditions reduce airflow around beans. Plants dry slowly and disease develops rapidly.
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