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Causal Agent:
BSR is caused by a residue-borne fungal pathogen, Cadophora gregata formerly known as Phialophora gregata. There are two pathotypes of the pathogen: 1 and 2. Pathotype 1 will cause symptoms on leaves such as chlorosis and necrosis, while pathotype 2 only causes browning of pith and vascular system. Both genotypes can be effectively managed using the strategies outlined in this fact sheet.
Signs and 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 characteristic 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 stem may have a “greasy” appearance.
Foliar symptoms may be absent, but when present consist of wilting, chlorosis, and necrotic tissue between the veins. These foliar symptoms are very similar to another soybean disease, sudden-death syndrome (SDS). The primary difference between these two diseases is the discoloration of the pith. BSR will have a brown to red pith, while in SDS it will remain white.
As disease progresses, it is possible that 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 pathotypes of C. gregata; 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.
Severe foliar symptoms of BSR
A healthy white pith (left) compared to a BSR-infected, brown pith (right)
Disease Cycle:
The fungus, Cadophora gregata, survives mainly on crop residue left on the soil surface. Conidia (spores) are produced in late spring, favored by cool temperatures, then the fungus invades roots and subsequently the vascular system. The fungus will colonize in the pith and vascular system, eventually moving to the leaves, if pathotype 1. Symptoms most commonly appear after pod formation.
Disease Management:
Host Resistance: Brown stem rot resistant varieties have been developed. The genetic source of brown stem rot resistance, however, is limited. This means that growers should not rely on resistant varieties, but use a combination of management practices to reduce the incidence and severity of this disease.
Cultural Practices: Continuous planting of soybeans will result in the build-up of inoculum on crop debris, which results in more disease and greater severity when environmental conditions are favorable. Rotating 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. A minimum of two years between soybeans crops in fields with a history of brown stem rot is recommended.
In no –till cropping, severity of brown stem rot was 30% greater and yields were 15% lower than in conventional tillage, according to a study completed in Wisconsin. When severe disease occurs, deep plowing of infested crop debris may reduce the survival of the fungus. This practice should be used in combination with sufficiently long rotation sequence for the best results.
Early-Maturing Varieties: 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 row does not influence the incidence of brown stem rot.
Soil pH: Brown stem rot severity is higher when pH decreases. Modifying soil pH to close to 7.0 will reduce the risk of Brown stem rot, except when SCN is present. Higher pH is associated with high levels of SCN, so soil pH should be maintained at 6-6.5 when both BSR and SCN are present.