This is the time of the year when stem canker can appear in soybean fields. Stem canker is favored by prolonged wet weather early in the season and by conservation tillage. It has recently been reported this season from east-central Illinois. Stem canker can be a serious problem under conditions that favor this disease. This article will cover some basic information pertaining to stem canker.|
Early symptoms of stem canker include slightly sunken, brown lesions at the base of leaf nodes. They are usually seen during reproductive growth, long after infections have occurred during early vegetative growth. Lesions can expand into elongated, sunken, reddish-brown cankers and may girdle the stem. Brown discoloration may also develop inside the stem, and plant parts above the lesions may die. The leaves develop necrosis and chlorosis between the veins and may remain attached after death. Lesions can be found at the soil line, making it possible to confuse this disease with Phytophthora rot. Stem canker, however, does not cause root rot, and the lesions lengthen down the stem, while the Phytophthora rot lesions begin on the roots and elongate up the stem. Small black dots may appear in cankers singly or in clusters after plants have senesced.
Stem canker on soybean stem.
Two different types of stem canker may occur in Illinois. Northern and southern stem canker are two similar diseases caused by different pathogens. Both diseases can cause yield reductions and kill plants, from flowering to the end of season. Northern stem canker is caused by the fungus Diaporthe phaseolorum var. caulivora, and southern stem canker is caused by Diaporthe phaseolorum var. merdionalis. Both of these pathogens can overwinter in infested soybean residue and may be spread with infested seed. Stem canker is closely related to pod and stem blight, which is caused by the related pathogen Diaporthe phaseolorum var. sojae. Pod and stem blight does not produce stem lesions.
Stem canker is best managed by planting resistant or moderately resistant varieties. Several resistance genes are known for southern stem canker, but resistance genes are not known for northern stem canker. Delayed planting may be helpful, and tillage may reduce disease in fields where stem canker has been a problem. Foliar fungicides such as Bravo, Quadris, and Topsin-M may also help to control stem canker. See the latest edition of the Illinois Agricultural Pest Management Handbook for additional information on the use of fungicides.--Dean Malvick