Leaf Blight on Bush Beans

Common Blight

• Common blight, caused by the strain Xanthomonas phaseoli and the sub-strain fuscous blight, caused by Xanthomonas phaseoli var. fuscans, can cause symptoms in the leaves, stems and fruit. Infected leaves turn brown and drop suddenly. Bush bean pods develop spots that turn into irregular patches if untreated. Pods may shrivel and seeds may not develop. A yellow, bacterial crust forms over infected portions of the bean plant during warm and humid weather.

Halo Blight

• Halo blight, caused by the pathogen Psuedomonas syringae, presents as light yellow to green circles around brown patches on bean leaves. These circles can be up to 1/2 inch in diameter. Untreated, the blotches can merge and the entire leaf will turn yellow and die. Halo blight can affect the stems and pods as well. Stem symptoms appear as long, reddish-brown lesions. Pods of bush bean plants affected by halo blight show similar symptoms to pods infected with common blight.

Prevention

• Prevention of blight diseases is more effective than treatment. Planting bush beans from clean, treated seeds obtained through reputable sources is the most effective preventative measure. Practicing crop rotation will also help to prevent blights. The bacteria causing blights can persist in the soil for up to two years. Rotations of bush bean planting on three-year cycles can help to prevent blights. Avoid planting and working in planting areas when the ground is wet.

Treatment

• Bush bean plants infected with blight diseases can be sprayed with a fungicide containing fixed copper at the first sign of infection. This treatment helps to prevent further spread of the infection. Applications can be made at seven- to 10-day intervals throughout the growing season up until one day prior to harvest. Affected plants should not be watered using overhead irrigation methods. Water with a soaker or drip hose to avoid moistening the leaf surfaces.