How to Manage Pests

UC Pest Management Guidelines

SYMPTOMS

White mold first appears as a watery rot on stems, leaves, and pods. White mycelium is often visible on the surface of rotted tissue under moist conditions. The development of black, irregularly shaped sclerotia is the best diagnostic feature. The disease may occur on the stem near the soil line (especially common in garbanzo beans) or more commonly, on pods, leaves, and stems. Affected tissue dries quickly and bleaches to a pale tan or almost white color. Entire branches or stems may be killed, which results in yellow flagging in the field. When the main stem is affected near the soil line, the entire plant may be killed.

COMMENTS ON THE DISEASE

Sclerotia survive in the soil for several years. After several weeks of moist and cool (39°F or 4°C) conditions, sclerotia within 2 inches of the soil surface are preconditioned to produce mushroomlike fruiting structures called apothecia, which form after one to several weeks of temperatures at 52° to 68°F (11° to 20°C) in moist soil. Airborne spores (ascospores) are then released from the apothecia by changes in relative humidity and germinate on plants parts, especially senescing flower parts. Ascospores commonly colonize senescent petals that are attached or detached. 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. White mold develops most rapidly at 68° to 77°F (20° to 25°C). White mold is generally more severe in fields with heavy canopies.

Other plant surfaces, especially the lower stem near the soil surface, appear to be susceptible, even before flowering. It is possible that some sclerotia germinate directly and infect the crown of a plant through mycelial invasion.

Sclerotinia sclerotiorum infects many cultivated plants and weeds. Ascospores may blow in from other fields and start epidemics in beans.

MANAGEMENT

Cultural Control
Rotation with nonhosts, such as small grains and corn, may reduce soil inoculum. However, many weeds are hosts and may maintain the fungus. Airborne ascospores may also blow in from surrounding fields. Avoid heavy applications of nitrogen to reduce excessive canopies. Wide row spacings may help keep the foliage dry.

Treatment Decisions
Chemical control is difficult because infection is often limited to plant parts covered by the leaf canopy. Apply fungicides during the flowering period to provide coverage on senescing petals.

PUBLICATION

UC IPM Pest Management Guidelines: Dry Beans
UC ANR Publication 3446
Diseases
R. M. Davis, Plant Pathology, UC Davis
Acknowledgment for contributions to Abiotic Disorders:
A. E. Hall, Botany and Plant Sciences, UC Riverside
Acknowledgment for contributions to virus sections in Diseases:
R. L. Gilbertson, Plant Pathology, UC Davis
Acknowledgment for co-authorship of Ascochyta Blight:
C. A. Frate, UC Cooperative Extension, Tulare Co.

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