The dry weather during the last two weeks of July has not been favorable for the development of soybean diseases, but bacterial leaf diseases have been noticeable by some growers. There are two common bacterial foliar diseases on soybeans in Iowa, bacterial blight and bacterial pustule, each caused by different bacteria.
||Bacterial blight on soybean.
Bacterial blight is caused by Pseudomonas syringae pv glycinea. New lesions of bacterial blight are small, angular, yellow to brown spots on leaves. The centers soon dry out, turn reddish brown to black, and are surrounded by a water-soaked margin bordered by a yellowish green halo, a feature that is lacking in brown spot. Young leaves are most susceptible to the bacterial infection, therefore, the disease is first noticeable on the top of the plant. The angular lesions enlarge in cool (70-80° F), rainy weather and merge to produce large, irregular lesions. The centers of old lesions often drop out, resulting in a ragged appearance of infected leaves. The bacteria also can infect stems, petioles, and pods.
Bacterial pustule is caused by Xanthomonas campestris pv glycinea, and the symptoms are very similar to those of bacterial blight. Symptoms of bacterial pustule are more pronounced in new leaves than old leaves. Early symptoms of bacterial pustule are many pale green minute spots on new leaves. Each spot later has a dark raised pustule on either surface of the leaf. The pustule at the center is more obvious on the lower surface. In Iowa, bacterial blight is more common in a growing season than bacterial pustule, which is not seen in most seasons. The higher than usual bacterial pustule incidence this year may be due to the warm weather in July because development of this disease requires high temperatures (85-90° F).
Bacterial foliar diseases, especially bacterial blight, are common minor diseases in Iowa and they have not been considered as a yield-limiting factor in production. In severe cases, bacterial foliar disease may cause some early defoliation, but it will not kill whole plants. Severely affected plants may have smaller seeds. Some varieties are resistant and some are not. Varieties that are susceptible to this disease may have more symptoms than other varieties.
It is important to monitor bacterial diseases in seed production fields because they are seedborne diseases and may affect seed quality in fields severely infested with such diseases.
This article originally appeared on page 163 of the IC-480(21) -- August 10, 1998 issue.