White Mold

White mold first appears as a watery rot on stems, leaves, and pods. White mycelium (threadlike hyphae) is often visible on the surface of rotted tissue under moist conditions. The development of black, irregularly shaped sclerotia (compact masses of mycelia) is the best diagnostic feature.

The disease commonly occurs on pods, leaves, and stems, but may occasionally occur at the base of the stem near the soil line (often in garbanzo beans). Affected plant 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 infected near the soil line, the entire plant may be killed.

Sclerotia survive in the soil for several years. Under the suitable soil temperatures and moisture in winter or spring, sclerotia can either germinate directly (by producing mycelia that infect plants when they come into contact with root tissue) or they can produce mushroom-like fruiting structures called apothecia. Apothecia produce airborne spores, which can lead to infection when they land on bean stems, leaves, or pods. Older, deteriorating (senescing) flower petals are very susceptible to infection. In the case of garbanzo beans, infection can develop during any growth stage.

White mold mycelium on the bean plant develops most rapidly at 68° to 77°F (20° to 25°C). White mold is generally more severe in bean fields with heavy canopies, which increase humidity. Infestations occur most often in cooler areas, such as the coastal regions, or on garbanzos grown in the Central Valley during the winter months. Common beans and limas can be susceptible as well if grown on the coast where there's higher moisture due to fog in the summer months.

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

Rotation with nonhosts, such as small grains and corn, may reduce soil inoculum. It is difficult, however, to prevent airborne ascospores from blowing into the field from host weeds or neighboring fields.

Use management practices that reduce heavy canopies and increase air movement:

• Avoid heavy applications of nitrogen, which can produce excessive canopies.
• Consider wide row spacing. This increases air movement in the canopy and may help keep the foliage dry, reducing favorable conditions for infection.

Plant garbanzo beans later (December through early February) rather than early (early November) since the extra growing time can result in heavy canopies that increase humidity and promote infestations.

Start inspecting plants for symptoms of white mold at emergence, especially for garbanzos which emerge in the cooler winter weather. Apply fungicides as needed. Spray coverage is important and often difficult since the disease can develop deep in crop canopies.