Stem Canker in Soybeans

Causal Agent

• Stem canker is a serious soybean disease and is further classified as northern stem canker (caused by Diaporthe phaseolorum var. caulivora) and southern stem canker (caused by Diaporthe phaseolorum var. meridionalis). The symptoms of both varieties of fungi are similar in nature. Left uncontrolled, stem cankers can reduce yields in soybean fields from 50 to 80 percent. It takes several years for losses to occur following infection.

Symptoms

• The disease starts with yellowing areas between the veins of the leaflets in the upper plant area. This is followed by light or dark brown lesion or cankers on the main stem near the soil line and at base of leaf nodes. Cankers gradually get more sunken, elongated and reddish brown in color. The rotting girdles the entire stem and kills the healthy plant areas above. Fungal reproductive spores or fruiting bodies are visible within the affected areas. Dead foliage remains attached to plant. Stem canker symptoms do not extend below the soil line to the roots.

Favorable Conditions

• Infections are more pronounced during periods of extended rain in early vegetative growth stage. Excessive moisture allows rapid spore germination in the soil and the rain and wind help with dissemination. Early infection causes more severe losses than late infections. Temperatures ranging between 82 and 93 degrees F also favor the spread of disease. Soybeans that are rotated with alfalfa are more prone to the infection, as the latter is also a fungus host.

Management

• Best management practices involve the use of resistant cultivars and inoculated seed. Do not save seed from infected plants. Clear fields of all debris and crop residues prior to planting and plow deep. The use of foliar fungicides is moderately effective only when applied at the vegetative state. Recommended fungicides include chlorothalonil, thiophanate-methyl or azoxystrobin (sold under various trade names).