Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu
Brown Stem Rot (BSR) is a fungal disease of soybeans that has never been positively confirmed in the Chesapeake Bay area, although suspect plants have been occasionally observed over the years. We have not observed significant levels of this disease, but it will be important to monitor going forward, especially with the quantity of no till agriculture and potential for reduced rotation lengths due to recent commodity prices.
BSR, as the name implies, causes a brown discoloration of soybean stems, which extend from the base of the stem up the plant (Figure 1).
Figure 1. BSR symptoms on the outside of the plant include a general browning that will extend from the soil line up the stem. Symptoms may progress with age.
When affected stems are split lengthwise you will observe a brown discoloration of the vascular tissue (Figure 2). Don’t confuse these with damage caused by stem-boring insects. In these cases, the insect pupae may be observed within the stem.
Figure 2. A comparison of a healthy soybean stem (top) with a stem exhibiting symptoms of BSR.
Foliar symptoms may or may not be observed after R3-R4. Foliar symptoms vary, but interveinal chlorosis, similar to what is observed for Sudden Death Syndrome (SDS), may occur (Figure 3)
Figure 3. Foliar symptoms of BSR may appear similar to SDS, compaction, or other issues encountered in soybeans. The key to identifying BSR is the presence of vascular discoloration in the lower stem around R5.
Unlike BSR, the lower stems of SDS-infected plants will not appear brown when split lengthwise. Occasionally foliage of BSR infected plants simply turn brown and remains on the plant, a symptom similar to aerial blight.
The fungus that causes BSR survives in soybean residue in the soil. Main roots of the plant are colonized early in development under persistent wet, and typically cool conditions. Foliar symptoms do not occur if air temperatures approach or exceed 95°F between R3 and R4. BSR symptoms tend to be more severe in fields with significant pressure from Soybean Cyst Nematode. There are two types of the fungus that vary in their aggressiveness and ability to cause disease on soybeans. Currently we are in the process of confirming the disease and typing this fungus.
Management of BSR is accomplished by encouraging residue decomposition and selecting resistant varieties. Rotation and sizing residue should help in this regard. Seed treatments and fungicides do not help with this disease.