Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu; @Delmarplantdoc
Stem canker is a disease that has been observed at increasing levels in many regions where soybeans are produced. In Delaware and Maryland we typically observe both Northern and Southern stem canker. Although different in some aspects, their overall impacts and management are similar. Both diseases are caused by fungi belonging to the Diaporthe complex.
Symptoms
As the name indicates, the hallmark of this disease is a canker that forms on the stem. Northern stem canker typically starts at a node within the canopy, and the canker spreads from the node over time. Symptoms of Southern stem canker are often observed during the reproductive phases of growth, and lesions typically occur in the lower portions of the plant. Cankers typically contain black lesions and sunken cankers with grey/brown centers. Cankers can restrict water and nutrient movement throughout the plant, which may impact yield.
Fungal structures forming on the lower stem of a soybean plant.
Sunken canker on the lower portion of a soybean stem likely caused by the Southern canker pathogen.
Infections occur early, during the vegetative growth phase of the plant. For Northern stem canker, foliage can prematurely wilt and often necrotic foliage remains attached to the stem. Plants affected by Southern stem canker may present foliage with interveinal necrosis, meaning that the veins remain green and the tissue between the veins turns yellow/brown. Foliar symptoms can resemble those caused by other soil borne diseases or issues, such as root knot or soybean cyst nematode infestation, SDS, and other diseases affecting the stem such as BSR, white mold, and Southern blight. In plants infected by Southern stem canker, spore bearing structures may form on highly susceptible cultivars. The pathogen is returned to the soil in/on infested residues, but some seed transmission may occur. Weeds such as morningglory, amaranth, hairy indigo, and wild poinsettia can also serve as hosts of the disease. Infected residue is by far the most important source of inoculum for disease.
The impact of the disease on plant yield depends on the age of the plant at the time of infection. Plants infected earlier will suffer greater yield impacts as the pathogen will have a longer period of time to infect, produce cankers, and restrict water and nutrient movement.
Management
Disease caused by stem canker can be minimized by 1) Managing residue and promoting residue decomposition. This includes tillage where practical; 2) Purchasing certified seed; 3) Planting cultivars resistant to stem canker. Plants with resistance to Northern stem canker may not be resistant to Southern stem canker and vice versa. In addition, because these diseases are spreading, it is not known if changes in the Diaporthe population may be impacting the effectiveness of resistant varieties. Research is currently underway to begin to answer these questions; 4) Planting double crop beans or delaying planting of full season beans. Fungicides are not effective in stem canker management and are not economical for disease management.