Scout for Lima Bean Downy Mildew

Emmalea Ernest, Extension Fruit & Vegetable Specialist;

Lima bean downy mildew is caused by Phytophthora phaseoli. Other Phytophthora species cause devastating disease in vegetable crops such as P. infestans, which causes late blight in potato and tomato and P. capsici, which causes fruit or stem rots in many vegetable crops (i.e. watermelon, cucumber, pepper, lima bean, pumpkin). While P. capsici can affect many crops, lima bean downy mildew (P. phaseoli) only infects lima bean. It does not infect other related bean crops, such as snap bean. Because of its limited host range, lima bean downy mildew has rarely been detected outside of the lima bean growing areas of Delaware or the Eastern Shore of Maryland.

In recent years, lima bean downy mildew has been well controlled in large acreage plantings using fungicides. This has reduced the risk of downy mildew for smaller scale growers and gardeners. However, downy mildew oospores can persist in the soil for several years and this season’s wet conditions are conducive to downy mildew’s development. I have seen one suspected incidence of downy mildew in pole lima beans so far this season. If you observe downy mildew in your lima beans, please contact me. I am interested in collecting samples for research purposes.

Downy mildew affects lima bean pods, racemes (flower stalks), new shoots and tendrils; it does not cause symptoms on mature leaves. Affected plant parts develop a cottony, white mat of mycelia (Figures 1 and 2) which will be more apparent under humid conditions. Later in the infection process the affected pods or racemes will appear dry and brown. Plants are most susceptible at the flowering and early pod set stage. Because it directly affects pods and racemes, downy mildew can cause severe yield loss.

Downy Mildew on Lime Beans 9-1-23Downy Mildew bottom of lima beans 9-1-23

Figure 1. Downy mildew symptoms on baby lima bean.

Pole Lima Bean pod with downy mildewPole Lima Beans with various symptoms of downy mildew

Figure 2. Downy mildew symptoms on pole lima bean.

Fungicide applications are only recommended at or after flowering when weather conditions favor disease. Preventative applications of a fixed copper fungicide or labeled phosphonate fungicide such as Phostrol, K-Phite, Agri-Fos or ProPhyt are effective at preventing downy mildew. Other fungicide options are listed in the Bean Section of the Mid-Atlantic Commercial Vegetable Production Recommendations. Labeled phosphonate fungicides can also be applied as curatives after downy mildew is observed in a field, but the best control is achieved through preventative sprays. Do not use copper fungicides alone for curative control. Phosphonate fungicides are the recommended control option for gardeners.