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.
Foliar symptoms of BSR are interveinal chlorosis and necrosis (yellow and brown discoloration) of leaves (Figure 1 and 2). Leaves can then curl and die. Inside the soybean stem, vascular and pith tissue turns brown (Figure 3). Foliar symptoms of BSR can easily be mistaken for sudden death syndrome (SDS), which is also being found in Iowa at this time (Figure 4). In order to tell them apart, 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.