Brown Stem Rot
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).
Figure 1. Foliar symptoms of brown stem rot. Credit: E. Byamukama
Inoculum & Management
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.
Figure 2. Brown vascular tissue is the diagnostic characteristic for brown spot. Credit: E. Byamukama
Frogeye Leaf Spot
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.
Figure 3. Frogeye leaf spot symptoms. Credit: E. Byamukama
Inoculum & Management
Frogeye leaf spot is caused by a fungal pathogen, Cercospora sojina. This pathogen survives on crop residue but inoculum can also come from the seed. Frogeye leaf spot can be managed through selecting resistant cultivars, crop rotation, tillage (where practical) to remove or reduce source of inoculum, and through application of a timely (R2-R5) fungicide.
The frogeye leaf spot pathogen has been reported to be resistant to strobilurin fungicides (quinone outside inhibitors, QoI – Group 11) in a few states. If a fungicide is used, fungicide rotation utilizing different modes of action or using a combination of different modes of action may help delay frogeye leaf spot fungicide resistance development.