Recently, dead or dying soybean plants with petioles and leaves still attached, as well as live plants with stem lesions, have been noted in some Michigan fields. Stem canker caused by fungi in the Diaporthe-Phomopsis complex are the causal agents. There are several species within this complex that can cause stem canker, pod and stem blight and Phomopsis seed decay. Back in the 1950s, there was a widespread outbreak of stem canker in the north central region that was believed to be associated with the widespread use of highly susceptible soybean cultivars.
Stem canker can easily be confused with Phytophthora stem rot. One of the key differences is the location of lesions. Phytophthora will produce a brown to black lesion on the stem that initiates from below the soil line, while stem canker will produce lesions that initiate around nodes, appear sunken and may girdle the stem. Stem blight caused by the Diaporthe-Phomopsis may also be evident as linear rows of black specs (fungal fruiting bodies) on the stem, and at maturity, Phomopsis-infected seeds may be noted, which will appear cracked and shriveled with a white, chalky mold.
For disease management, use resistant or moderately resistant cultivars. If tillage is an option, it will reduce the survival and spread of spores onto next year’s crop. Use certified seed free of Phomopsis—seed treatments except metalaxyl/mefenoxam may provide some protection, particularly of seedborne inoculum.