Ohio State University Fact Sheet

Plant Pathology

2021 Coffey Road, Columbus, OH 43210-1087

Brown Stem Rot of Soybean

AC-35-98

Anne E. Dorrance
Patrick E. Lipps

Brown stem rot has increased in incidence and severity in Ohio in the past few years. This disease occurs most commonly in fields that have been in continuous soybeans, short rotations, reduced tillage or no-till. Environments that favor high yield potential also favor brown stem rot development. Under high disease severity, yield reductions from 10 to 38% have been reported. This soybean disease is caused by a residue-borne fungal pathogen, Phialophora gregata.

Symptoms

The most common symptom of brown stem rot is the brown to reddish-brown discoloration of the soybean stem pith. This internal stem browning is the diagnostic character of this disease, but the discoloration may only occur in sections of the stem. When disease is severe, the discoloration is continuous throughout the stem from the base of the plant upwards, and the outside base of the stems have a "greasy" appearance. Foliar symptoms may be absent, but when present consist of wilting, chlorosis and browning of the tissue between the veins. These foliar symptoms are very similar to another soybean disease, sudden-death syndrome. With brown stem rot, the leaves will wilt and eventually die. Foliar symptoms are influenced by many factors including; the soybean cultivar, temperatures, plant age, soil moisture and variation in the pathogen. There are two types of brown stem rot fungi; pathotype 1 causes foliar symptoms and greater yield losses in susceptible soybean cultivars than pathotype 2, which only causes internal browning in the pith. In addition, foliar symptoms may fail to develop if seasonal precipitation is below normal. When rain or irrigation follows flowering, foliar symptoms tend to be more severe in infected plants. Above normal air temperatures are reported to be suppressive to development of foliar symptoms.

Figure 1. One of the best ways to identify brown stem rot of soybean is to split the lower stem of affected plants to check for the brown pith region. Healthy plants will have a white pith.

Disease Cycle

The fungus, Phialophora gregata, survives mainly on crop residue left on the soil surface. Conidia (spores) are produced in late spring then the fungus invades roots and subsequently the vascular system. After pod formation, symptoms of brown stem rot can be found in affected plants. Low soil moisture reduces the severity of stem and foliage symptoms produced by pathotype 1.

Figure 2. Leaf symptoms of brown stem rot are quite charateristic, however, they do not always occur. Brown interveinal tissue with yellow to green veins are typical symptoms on the leaves.

Disease Management

Tillage, rotation and soybean cultivar all affect the severity of brown stem rot.

1. Crop rotation with non-host plants like corn and small grains will prevent build-up of the brown stem rot fungus to levels that cause economic losses. Continuous planting of soybeans will result in the build-up of inoculum on crop debris which results in more disease and greater severity in seasons with environmental conditions favoring disease development. A minimum of two years between soybean crops in fields with a history of brown stem rot is recommended.

2. Severity of brown stem rot was 30% greater and yields were 15% lower in no-till than in conventional tillage in a study completed in Wisconsin. When severe disease occurs, deep plowing of infested crop debris may reduce the survival of the fungus, but this practice should be used in combination with sufficiently long rotation sequence.

3. Brown stem rot resistant varieties have been developed. The genetic source of brown stem rot resistance, however, is limited. It is not recommended that growers rely only on resistant varieties, but use a combination of management practices to reduce the incidence and severity of this disease.

4. Additional disease management strategies for fields with a history of brown stem rot may include planting soybean cultivars with shorter relative maturity. These early maturing varieties may escape the yield reducing effects of brown stem rot in comparison to cultivars with later maturity or planting later in the season. It should be noted that planting soybeans in narrow rows does not influence the incidence of brown stem rot.