Shown in the photo is a disease called halo blight,
caused by the bacteria, Pseudomonas
syringae pv. phaseolicola. It is spread from leaf to leaf and plant to
plant in windblown rain or irrigation water. The bacteria enter the plant
through wounds and natural openings when the foliage is wet and during periods
of high humidity. The disease affects the seed pods as well as the foliage.
The first symptoms are small lesions resembling tiny pinholes on the underside of leaves. They rapidly dry and turn reddish brown and become visible on both sides of the leaf. These spots usually remain small, but a characteristic yellow zone, resembling a halo, often develops around the lesions. They can be of irregular size and shape. Symptoms on the bean pods begin as tiny spots gradually enlarging to form dark sunken lesions. Infected pods are discolored and shriveled.
Control: There is some resistance to this blight. For instance, navy and small white bean cultivars along with some red kidney bean cultivars are resistant to halo blight. Check for resistant varieties in the type of bean you desire. Since the disease is spread by rain, it would help to provide some type of cover that would lessen water droplets falling on the plants. Although these products will not eradicate the disease, copper based bactericides can reduce infection and protect healthy tissue. Do not plant beans in the same location for 3 years.
