Diseases of bean (
White mold
White mold of bean is caused by the fungus Sclerotinia sclerotiorum. S. sclerotiorum has a wide host range. In Arizona it causes losses in lettuce and dry beans as well as landscape ornamentals and vegetables. Disease is favored by cool, moist conditions. Dry beans grown under center pivot are most vulnerable since constant moisture and a dense succulent canopy are the best conditions for rapid infections. Initial symptoms on bean usually appear just after flowering and when the canopy closes. The fungus enters senescent flowers easily, and this point of entry is thought to be an important part of the disease cycle in beans. Water soaked areas develop on stems, pods and flowers, and characteristic white mycelial growth of the fungus develops. Black sclerotia develop on stems and pods (photo 1). The sclerotia are easy to see and are diagnostic of the disease. From a distance, symptoms in the field may look like small areas of wilted plants similar to water stress, but plants are rotten.
Sclerotia harden and can survive in the soil or plant debris for years. They can germinate and infect plants directly, or they may produce the sexual reproductive structure, an apothecium. Apothecia (photo 2) are small cup-shaped structures in which numerous ascospores develop. The ascospores are easily airborne and can infect the upper parts of the plant such as the senescent flowers.
White mold is controlled on bean by using both cultural methods and chemical applications. There is little tolerance to S. sclerotiorum in dry bean varieties, but using varieties that have more upright growth and wider spacing between plants will reduce moisture in the canopy and therefore fungal growth. Applications of effective fungicides are highly recommended for fields with a history of disease. Thiophanate methyl (Topsin-M on bean) and Iprodione (Rovral on snap, dry and lima bean) are effective. Other effective chemicals may have more recent registrations, but be sure to check current labels for all fungicides. Contans, a biological control product containing a mycoparasitic fungus, may also be useful for eradication of sclerotia when applied to fields before planting.
For more information see: http://www.ext.colostate.edu/PUBS/crops/02918.html
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March 3, 2009