The conditions this year have resulted in a number of soybean diseases starting early and northern stem canker is one such disease. In most years, stem canker symptoms become apparent later in the seasonafter flowering. This year stem canker started earlier with seedling damping off and early season wilting symptoms which is as stated above a response to the growing season.
What does Stem Canker look like?
Symptoms are often described as a general yellowing of the top leaves of the plant with dark reddish-brown sunken cankers at the nodes. The lesion may extend several inches, often on one side but does not usually extend down as far as the soil line (different then Phythophthora wilt). In severe cases however the lesion may cover the entire length of the stem. A cross section of the stem will reveal a slight browning at the nodes at first followed by complete disintegration of the stem in severely infected plants. The sudden wilting of the plants and the stem canker can be confused with Phytophthora root rot.
Besides the yellowing of the leaves and stem lesion, stem canker can cause a “top dieback” especially later in the season.
In this case, the upper four to six internodes or branches of the plant become dark brown and as the name implies the top dies (wilts). Soybean plants may become more susceptible as the plants go through physiological changes due to flowering.
The fungus responsible for northern stem canker (Diaporthe phaseolorum var. caulivora) survives in crop residues and is therefore, influenced by not only rotation but tillage management practices. It is suspected the re-emergence of stem canker in the northern U.S. and Ontario is due not only to more soybeans and less tillage but possibly an increase in susceptible varieties being grown.
Fungicide seed treatments have been shown to reduce early infection or introduction but will not help with later season infections. Foliar fungicides can be effective but again results have been inconsistent. Crop rotation with non hosts such as corn and wheat (small grains) can reduce the disease in the future as well as planting a resistant variety. Reducing the amount of infested soybean residue on the soil surface is another management tool to consider.