There are two similar but distinct stem canker diseases. Northern stem canker is prevalent in northern regions. Southern stem canker is prevalent in the southern United States but has been reported in Wisconsin and Illinois as well. Infected plants usually occur in patches within the field. Initial symptoms are small, reddish-brown lesions that appear soon after flowering near the nodes in the lower third of the soybean canopy. Lesions expand longitudinally and form sunken cankers that are gray-brown with reddish margins. Stem canker can be confused with Phytophthora stem rot; to distinguish them, stem canker has green stem tissue below the canker and does not cause root rot. Interveinal foliar chlorosis and necrosis may occur as a result of a toxin produced by the fungus. These symptoms can be similar to those of brown stem rot and sudden death syndrome.
The fungus survives in infested residue or in the soil for several years. Infection occurs when spores are splashed by rain onto plants in early vegetative growth stages. Northern stem canker is believed to be associated with cooler temperatures and extended periods of rainy weather occurring early in the growing season. Southern stem canker infection requires 24 to 96 hours of leaf wetness and warm temperatures (70 to 85ºF).
Variety selection: Varieties with resistance to northern stem canker are available.
Tillage: Incorporation of infested crop residue into the soil will reduce survival of Diaporthe species. If tillage is an option, use proven conservation tillage practices to reduce erosion and maintain soil quality.
Seed treatments: Fungicide seed treatments, except for metalaxyl/mefenoxam, may reduce stem canker.
* Southern stem canker is seed transmitted, but northern stem canker is not known to be seed transmitted.
Photo by Adam Sisson