White Mold

Sclerotinia sclerotiorum

This fungus disease can affect a wide variety of crop and weed plants, but is best known as a pest of beans. It is most common in cool humid areas.

White mold usually infects beans though the flowers, though it can also enter through wounds. The spores germinate on the petals and get established and then the fungus spreads to the pods, stems and leaves. The characteristic symptoms are water soaked lesions which are eventually covered in white fungus (within a couple of weeks). In serious cases it can kill all of the leaves. The good news is that because it needs flowers to get established, it usually only occurs within a couple of weeks of flowering.

Spores can survive over the winter in the soil, or on crop debris, so it is important to clean up crop debris and removing infected plants. It requires moisture to get established and can be kept under control by keeping the plants dry (use drip irrigation or water early in the day so plants can dry out quickly). You should also have good air circulation and should practice good crop rotation.

Image: Howard F. Schwartz, Colorado State University, Bugwood.org