Soybean diseases are starting to show up in Iowa fields this growing season. In an On-Farm Network trial in north central Iowa, brown stem rot (BSR) was recently found. This disease is caused by the fungus Phialophora gregata, and infection can result in yield loss for Iowa farmers. The following information and scouting guidelines are provided by Daren Mueller, an Iowa State University Extension plant pathologist. Adam Sisson, ISU Integrated Pest Management specialist, and Tristan Mueller, an agronomist with the Iowa Soybean Association's On-Farm Network.
Foliar symptoms of BSR are interveinal chlorosis and necrosis (yellow and brown discoloration) of leaves (see photos 1 and 2). Leaves can then curl and die. Inside the soybean stem, vascular and pith tissue turns brown (see photo 3). Foliar symptoms of BSR can easily be mistaken for sudden death syndrome (SDS), which is also being found in Iowa at this time (see figure 4). In order to tell them apart, you need to split the stem and look at the pith tissue. If the pith is brown and not the normal white color, then the plant likely has BSR. Not all BSR-infected plants show foliar symptoms, however.
Photo 1. Soybean plant showing foliar symptoms of brown stem rot.
Photo 2. Leaf of brown stem rot infected plant showing necrosis and chlorosis of tissue.
Photo 3. The internal stem browning of plants with BSR can help distinguish it from SDS.
Photo 4. Foliar symptoms of SDS.
What can farmers do to manage brown stem rot disease? During the growing season nothing can be done to control BSR, but is important to know if it is present for future management. The main control of BSR is selecting soybean varieties that have resistance. Also, crop rotation and tillage can help reduce BSR. Soybean cyst nematode (SCN) often occurs with BSR, and SCN may "break" the ability of resistant varieties to reduce disease.
* Daren Mueller is an assistant professor in the Department of Plant Pathology and Microbiology; he can be reached at 515-460-8000 or e-mail [email protected].
* Adam Sisson is an Integrated Pest Management extension specialist; he can be reached at [email protected] or 515-294-5899.
* Tristan Mueller is the Operations Manager for Iowa Soybean Association's On-Farm Network and can be reached at 515-334-1075 or e-mail [email protected].