No. 16/July 10, 1998
Septoria Brown Spot of Soybeans
We have received several calls during the past few days concerning the presence of Septoria brown spot disease on soybeans. This fungal pathogen is commonly seen only during the latter parts of the growing season, when plants are almost fully mature and beginning to drop leaves.
With warm and humid conditions occurring throughout Illinois, brown spot is appearing earlier than normal. The fungus, Septoria glycines, requires very high moisture to release spores. The sporulation structure absorbs water and swells; spores are then pushed out the top to infect adjacent leaf tissue. It is also not a very aggressive parasite, which accounts for infections being seen primarily on the older, lower leaves. However, if conditions are favorable, the fungus can infect lower leaves on young plants, as well as becoming a seedling blight. This pattern is especially true when plants are growing under stress, such as higher than normal rainfall, wet soils, reduced sunlight, etc. These stresses can reduce the ability of plants to resist pathogens and also favor the growth and development of certain diseases.
Control of brown spot is not usually necessary in Illinois. In some years, research has shown a yield increase of between 0 and 12 bushels per acre, depending on disease severity and location within the state. Applications of any common foliar fungicide provide control of brown spot but also add an additional cost to production. Applications made early in the season have not typically shown a yield improvement when compared to later applications during podfill stage. Therefore, unless the disease levels are severe at this time, it would be more economical to wait until later in the season when an economic return is much more likely, especially if the rainy weather patterns continue.
H. Walker Kirby (firstname.lastname@example.org), Extension Plant Pathology, (217)333-8414