Brown Stem Rot And Frogeye Leaf Spot Developing In Soybeans

 

 

Brown stem rot (BSR) was found in a few soybean fields in Brookings and Moody counties the week of September 4 at very low incidence. BSR is a late season disease and its symptoms can look like sudden death syndrome. Both diseases cause interveinal yellowing of leaves and necrotic blotches between the veins (Figure 1). The best way to tell BSR from other stem rots is to split the soybean stem lengthwise and inspect it for vascular tissue discoloration. BSR causes the vascular tissues to be brown (Figure 2).

 

 

 

Brown stem rot is caused by a fungal pathogen, Phialophora gregata. This pathogen survives in soybean stem residue and in soil. Soybean infection by the BSR pathogen takes place through the main and lateral roots and slowly grows upward in the vascular tissues. Infected plants do not show symptoms until later in the season after the R3/R4 (beginning pod/full pod) growth stage.

 

Brown stem rot can be managed through the use of resistant varieties. Select soybean varieties which also have SCN resistance for fields with a history to both SCN and BSR. Tillage and rotation can reduce the inoculum of BSR. Longer rotations may be needed for fields with high BSR incidence.

 

 

 

Frogeye leaf spot was also found in several soybean fields scouted in east central and southeast South Dakota counties. Frogeye leaf spot is characterized by irregular to circular lesions which are tan to gray in color with reddish-purplish borders (Figure 3). This disease usually develops in soybean late in the season in our area and infection is favored by frequent rains and warm conditions.