There are no external symptoms of brown stem rot until after pod development begins. Even though browning of the internal stem and root systems of infected plants are present as early as the V4 growth stage, there is no external evidence of BSR at this time and signs of early infection will go unnoticed unless the stems are cut open and examined. Infection is evident as a dark reddish-brown discoloration of the stem pith, which begins at the lower portion of the stem (Figure 1). Stem infection intensifies as the plant begins to flower. At this time, the brown stem rot fungus moves out of the vascular system into the stem pith. Discoloration of the pith may eventually extend to the tips of the infected plants.

Figure 1. Browning of the internal stem (left) is diagnostic for BSR. The middle stem may be developing symptoms. Compare to healthy, white pith in the stem on the right.

Foliar symptoms generally appear in August and are more severe with dry soil conditions. These symptoms may start as a gradual yellowing followed by wilting, curling and death of leaves (Figure 2).

Figure 2. Early leaf symptoms of BSR

Tissues between veins become yellow and quickly turn brown, except for a narrow band of green tissue outlining the vein (Figure 3). Not all susceptible soybean varieties express the "classical" foliar symptoms, however. In some cases leaves may suddenly die out and remain attached to stems. Foliar symptoms of brown stem rot may be misdiagnosed as frost damage, early maturity, or wilt due to dry soils. Yield loss is generally greatest when foliar symptoms develop, compared to when symptoms are evident only inside stems.

Figure 3. Foliar symptoms of BSR. Note that necrotic (brown, dying) tissue is between the veins, while the vein tissue remains green.

Foliar symptoms can be suppressed by high temperatures during R3 and R4 growth stages, and are most severe if optimal soil moisture occurs after flowering, followed by hot, dry weather. The presence of foliar symptoms can also be influenced by the pathotype of P. gregata present in the field.

L eaf symptoms of BSR don't always, occur, may occur for only a few days, or may be misdiagnosied as frost or drought injury.