May 24, 2018
Septoria brown spot is a common disease of soybean in the Midwest. Significant yield loss from Septoria brown spot is rare. Septoria is most common when the environment and plant leaves are wet and warm for an extended period of time. Continuous and minimum tillage soybean fields are more likely to be infected.
What to Watch For
Septoria brown spot (Septoria glycines) is a common soybean disease in the Midwest and is associated with wet weather. This fungal disease overwinters in soybean debris and is more likely to be an issue in continuous and minimum tillage soybean fields. During rain events, spores are splashed from the debris up into the lower soybean canopy where they are able to infect and cause Septoria brown spot symptoms. Hot, dry weather generally stops the spread of the pathoge
Impact on Your Crop
Septoria brown spot symptoms consist of small, brown, irregular spots that can turn the leaves yellow and cause premature leaf drop. Smaller seed size can also result from infection. Symptoms can occur on both upper and lower leaf surfaces, including the first true leaves if infection occurs very early. This disease is common in the lower leaf canopy and is generally thought to have minimal impact on yield potential. Heavy and frequent rainfall later in the season can cause spores to be splashed up into the upper canopy, allowing disease to develop on upper leaves. Severe infection can result in a yield loss of 5 to 8 percent.1
Bacterial blight can be confused with brown spot. Bacterial blight lesions are angular, brownish, surrounded by a yellow ring or halo, and may appear water-soaked. The lesions turn dark as they mature and drop out of the leaf, giving the leaf a tattered appearance. The spore-producing specks that are characteristic to brown spot are absent with bacterial blight.3
Tips to Manage
Individual soybean products can vary in their susceptibility to Septoria brown spot; resistant products are not available. Foliar fungicides applied at R3 and R6 growth stages can offer some control in the rare case that a severe infection appears to be developing.2 Tillage and rotation to non-legume crops may help decrease incidence.
Figure 1. Septoria brown spot (left). Bacterial leaf blight (right).
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Sources
1 Malvick, D. Septoria brown spot. Crop Diseases. University of Minnesota Extension. www.extension.umn.edu/.
2 Pedersen, P. 2006. Brown spot—Septoria leaf blight. Insects and Diseases. Soybean Extension and Research Program. Iowa State University. http://extension.agron.iastate.edu.
3 Wise, K. 2015. July 16,2015. Septoria brown spot vs bacterial blight in soybeans. Purdue Pest and Crop Newsletter. Issue 16. http://extension.entm.purdue.edu.
Web sources verified 05/09/2018.