NPIPM:Brown stem rot of soybean
Compiled by: Buyung Hadi, from the materials by: Loren Giesler, Dean Malvick and Lawrence Osborne
Brown stem rot is a common disease in the northern tier of the United States’ soybean growing regions, although it is considered rare in most parts of North Dakota. The disease can be yield limiting, especially in areas where shorter crop rotation and reduced tillage are practiced.
Causal Organism
Brown stem rot on soybean is caused by the fungus Phialophora gregata. Two different types of the fungus (type A and type B) have been documented in the Northern Plains. Type A of P. gregata causes severe foliar symptoms and internal stem discoloration; hence it is usually associated with higher yield loss. Type B is also associated with internal stem discoloration but it causes no or minor foliar symptoms.
Symptoms and Signs
The disease causes progressive browning of the pith and vascular issues. Browning in the pith usually begins at the nodes near the soil line and eventually fills the internodal areas. Internal browning can be clearly seen by splitting the lower stem vertically. Plants can develop stem symptoms without showing foliar symptoms. Typical foliar symptoms include yellowing and browning between the veins, leaf curling, and premature defoliation. Foliar symptoms usually occur during the mid- to late- reproductive stages of soybean development (R4-R6). Hot summer conditions can suppress foliar symptom development. Sudden death syndrome or stem canker may cause similar foliar symptoms, but the discoloration of pith tissues is unique to brown stem rot. Sudden death syndrome or stem canker may cause similar foliar symptoms, but the discoloration of pith tissues is unique to brown stem rot.
Disease Impacts
Depending on the environmental conditions, soybean varieties and fungal strain involved, brown stem rot of soybean may cause 9 to 44% yield reduction.
Life Cycle and Epidemiology
Phialophora gregata overwinters as a saprophyte on soybean debris in the field. While overwintering, the fungus is capable of asexual reproduction, contributing to the primary inoculum level in the next season. The fungus infects new soybean plants through the roots and stems near the soil level early in the season. The fungal mycelium grows within the xylem and may disrupt water and nutrient transport in the plant. Cool and wet weather are optimum for fungal growth. Disease development is optimum when the temperatures range between 60 and 75˚F. Temperatures higher than 80˚F suppress disease development. High soil moisture in the beginning of the season and low moisture through later reproductive stages are favorable for diseases development. Soybean cyst nematode can exacerbate the damage due to brown stem rot.
Management Approaches
Scouting
The symptoms of brown stem rot can be confused with sudden death syndrome. It is important to identify the disease properly in order to make an accurate management decision. Below is a table of diagnostic symptoms and signs to differentiate the two diseases:
| Plant part | SDS | Brown stem rot |
|---|---|---|
| Roots | Smaller than normal, necrotic, blue fungal masses often visible on the surface of the roots | Healthy, no blue fungal masses on the surface of the roots |
| Interior stem | Browning of vascular tissue, pith tissue remains white | Browning of vascular and pith tissues |
Cultural Methods
Avoid planting soybean in severely brown stem rot-infested fields for at least three years. Non-host crops such as corn, small grains and forage legumes can be used as rotation crops. This non-soybean period should lower the inoculum level of Phialophora gregata in the given field. In no-till systems, longer crop rotations should be used and a shredder can be used to break up soybean straw and reduce inoculum.
Host Plant Resistance
Soybean varieties vary in their susceptibility to brown stem rot. Usage of resistant soybean varieties is the most economical way to reduce yield loss associated with the disease. Thus, resistant soybean varieties should be used in fields with brown stem rot history. Since soybean cyst nematode can interact with brown stem rot, soybean varieties resistant to soybean cyst nematode may also help to reduce the yield loss due to brown stem rot. Soybean varieties should be rotated to maintain the resistance effectiveness.
