As soybeans start blooming, two foliar diseases start to appear in Iowa soybean fields. The diseases, bacterial blight and brown spot, are two of most commonly seen diseases. Already, we have received questions on these two diseases.
Bacterial blight can be found in almost every soybean field in any year. Bacterial blight is caused by Pseudomonas syringae glycinea. This disease sometimes can be misidentified as bacterial pustule, a disease that occurs mainly in the South. 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 brown spot does not have. Young leaves are most susceptible to the bacterial infection; therefore the disease is first noticeable on the top of the plants. The angular lesions enlarge in cool, 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. If pod infection occurs, bacterial blight can become seedborne.
||Bacterial blight on soybean.
During a growing season, the bacteria are dispersed by rain and wind. The disease is more frequently observed in areas that have received more rain. This week, I have observed a lower than normal occurrence of this disease. The disease is unlikely to cause significant damage in the rest of season.
Brown spot, also known as Spetoria leaf spot, is a fungal disease caused by Septoria glycines. The fungus normally infects aged leaves in the lower portion of the plants. Symptoms of the disease are many irregular, dark brown spots on both upper and lower leaf surfaces--different from bacterial blight symptoms. Adjacent lesions frequently merge to form irregularly shaped blotches. In the early stage, lesions of bacterial blight have a yellow halo, but brown spot does not. Brown spot lesions are chocolate brown to blackish brown in color.
||Brown spot on soybean.
Like bacterial blight, brown spot occurs in Iowa every year in every field. The brown spot pathogen also survives in crop residues and can be seedborne too. In a wet season, it is also dispersed from the soil to soybean plants by splashing rain. Normally it causes no significant yield losses unless premature defoliation occurs. Frequent rainfall is the primary condition for the occurrence of an epidemic. In a wet summer, the disease progresses rapidly from lower leaves to upper leaves. Because the brown spot pathogen infects aged leaves, soybeans weakened by other diseases or improper farming practices become susceptible to this disease. It has been observed that relatively high levels of brown spot occur in fields with severe soybean cyst nematode damage. Soybeans with Fusarium root rot also have more brown spot. If you find brown spots in plants of abnormal growth, check whether there is other primary cause, such as nematodes. You would be more likely to see this disease in continuous soybean fields or no-till soybean fields.
||Comparison of brown spot (left) and bacterial blight (right) in soybean.
Although severe damage by the two diseases is unlikely this season, the following information for management is provided in case you do need it:
- Use varieties with tolerance to the two diseases. There is no resistance available, but field tolerances have been observed. You should avoid using susceptible cultivars.
- Do not save seed from severely infested fields.
- Completely bury soybean residue by clean tillage, which will reduce disease risk significantly.
This article originally appeared on pages 146-147 of the IC-478(18) -- July 21, 1997 issue.