Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu
With the return of significant rain in most areas, the threat of downy mildew and white mold increases. Be sure to scout regularly for symptoms of these two important diseases of limas. See Kate Everts’ article, White Mold on Lima and Snap Beans, in this issue of WCU for more information on white mold.
For downy mildew the best predictor, for most situations, is the Hyre-Cox model that states that conditions for downy mildew are favorable when fields receive 1.2 inches or more of rain within 7 days and the average daily temperature during that period is 78°F or less. Heavy dew and/or fog reduce the amount of rainfall needed to provide favorable conditions. Fungicide recommendations for control of downy mildew in lima beans were discussed in the article titled Downy Mildew on Lima Beans in WCU 16:23.
Downy mildew on limas is characterized by white downy growth on the pods, petioles and racemes. A reddish brown border often surrounds the infected area on the pods.
Downy mildew on petioles and stems often produces the distorted “crooks” seen in this picture.