Stem canker of soybean
Multiple fungi in the genus Diaporthe cause stem canker. The first symptoms of stem canker are often dead plants with dried leaves that remain attached to petioles late in the season. Diseased plants usually occur in patches within fields. Stem lesions start as small, reddish-brown spots at the base of a branch or leaf petiole and then expand to form slightly sunken cankers that are reddish-brown with reddish margins. Cankers may remain on one side of the stem and can extend over several nodes or girdle the stem, killing the plant. In some cases, Diaporthe fungi cause top dieback by forming a dark brown canker on the upper four to six internodes, which kills only the top of the plant. Diaporthe fungi may also be associated with gray streaking in the lower stem and taproots. Interveinal foliar chlorosis and necrosis can occur as a result of a fungal toxin, but it is difficult to distinguish foliar symptoms from several other soybean diseases that cause similar symptoms.
The fungi that cause stem canker survive in the soil and in infested crop residue. Extended wet weather occurring early in the season is thought to favor disease. Infection occurs when spores are splashed by rain onto plants in early vegetative growth stages.
The best way to manage stem canker is to plant resistant soybean varieties. Rotating crops and incorporating infested crop residue into the soil will reduce the amount of inoculum available to infect the next soybean crop.
Gallery Images: D. Mueller, T. Allen, A. Sisson, K. Wise, and C. Grau