Nathan Kleczewski, Extension Specialist – Plant Pathology; firstname.lastname@example.org; @Delmarplantdoc
During a recent survey of soybeans in Delaware, we saw a few fields with target spot. This disease occurs at low levels in many fields in Delaware, and only rarely have we observed it at noticeable levels. The disease is caused by the fungus Corynespora cassicola, which overwinters in soybean residue and can survive in the absence of a host for roughly two years. Symptoms typically occur in the lower canopy, after canopy closure, due to high humidity requirements by the fungus to infect tissues. Lesions start as small specks that expand to large round or oblong spots with distinctive zonation, hence the creative name. The disease is typically limited to the lower canopy.
Target spot is not considered to be a disease that causes yield impact, and recent research from Southern soybean production areas, where disease incidence and severity is far greater than we have in Delaware, supports this fact. See this link for a detailed report from my colleague Tom Allen at Mississippi: http://www.mississippi-crops.com/2017/04/01/managing-target-spot-of-soybean-2016-fungicide-trial-efficacy-results/. If you are noticing target spot increasing in incidence or severity in your fields, the best thing you can do is manage residue and rotate away from soybeans for a year or two. Many soybean varieties are very tolerant to this disease.