Most of the soybeans are turning color, and leaves are dropping indicating beginning of natural senescence. However, some portions of the field may drop leaves early due to different stresses like salt toxicity, soybean cyst nematode, foliar diseases and root/stem rots. One of the stem rots observed on soybeans this week is brown stem rot (BSR).
Brown stem rot is caused by a fungal pathogen, Phialophora gregata. This pathogen survives in soybean stem residue and in soil. Infection of soybean by the BSR pathogen takes place through main and lateral roots and slowly grows upward in the vascular tissues. Infected plants do not show symptoms until late in the season after R3/R4 growth stage. Symptoms of BSR include interveinal yellowing and browning (Fig. 1). The most diagnostic symptom of BSR is browned pith in the stem (Fig. 2).
Fig. 2. Browning of vascular tissues caused by brown stem rot pathogen.
Brown stem rot symptoms can be confused with those of another fungal disease, sudden death syndrome (SDS) of soybean, or mistaken for a sign of maturity. To distinguish BSR from SDS, split the stem longitudinally and look for browned vascular tissue. It is thought that BSR severity may be increased by soybean cyst nematode presence.
Brown stem rot can be managed through use of resistant varieties. Select soybean varieties that also have SCN resistance for fields with a history of both SCN and BSR. Tillage and rotation can reduce the inoculum of BSR. Longer rotations may be needed for fields with severe BSR.