Plant Health Initiative

Brown Stem Rot - Symptoms

Internal stem browning is diagnostic for brown stem rot. Photo © Craig Grau, University of Wisconsin Click on image to view larger version.

Browning between the leaf veins is also a classic BSR symptom. However, it does not always occur in infected plants. Photo©Craig Grau, University of Wisconsin. Click on image to view larger version.

Recogizing brown stem rot is a major problem. There are no symptoms of brown stem rot until after pod development begins. At that time, internal stem browning is evident in infected plants if the stems are split longitudinally. This is a diagnostic symptom of BSR.

Wilting, premature defoliation and lodging may also occur. These symptoms are heightened when infected plants are subjected to drought stress.

The internal browning and infection can be severe in the absence of foliar symptoms. However, yield loss is generally greatest when foliar symptoms develop, compared to when symptoms are only evident inside stems.

The factors that control symptom development are under study , and probably include host resistance, temperatures, soil moisture and different pathotypes of P. gregata.

Foliar symptoms

Although BSR occurs commonly in the North Central states, the classic foliar symptoms of browning between the leaf veins are seen only sporadically. If foliar symptoms do appear, it is late in the growing season and are often confused with early crop maturity or the effect of dry soils.

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 suppress development of foliar symptoms. 

Look-alike diseases

BSR may be mistaken for sudden death syndrome (SDS) or stem canker because these diseases show similar leaf symptoms. However, SDS has symptoms on both leaves and roots. BSR and stem canker do not cause root rot.  The brown discoloration of the pith is also diagnostic for BSR, but not for SDS.

BSR Pathotypes

Pathotypes of P. gregata appear to vary in their pathogenicity. Two types have been distinguished genetically and in the severity of diseases they cause. Pathotype "A" (defoliating pathotype) causes more severe internal stem symptoms and defoliation than the other pathotype, called Pathotype "B" or "nondefoliating".