Tar spot of corn was confirmed in Michigan’s Allegan County within nonirrigated cornfields. Michigan State University researchers have confirmed the causal agent to be Phyllachora maydis. In Mexico, there is an additional species, Monographella maydis, which forms a complex with P. maydis to cause greater yield loss. So far this additional species has not been confirmed in the U.S.

In 2015, tar spot was found in the U.S. for the first time in Indiana and Illinois. Tar spot symptoms were observed in Michigan in 2016. MSU Extension reports that given how new this disease is, it is unknown what impact it will have on yield.

It is not known at this time if any particular management strategies will need to be implemented. However, if growers have an infested field, they are advised to consider cleaning the combine before moving to a new field, as moving debris may move the pathogen.

Michigan State University Extension educators say growers should keep an eye out for this disease during harvest. Spore-producing structures are produced by the tar spot fungus on the leaves, and as the name suggests, they resemble black spots of tar that feel bumpy to the touch and cannot be easily rubbed off.

Any grower who suspects tar spot is asked to submit a sample to MSU Diagnostic Services free of charge, courtesy of the Corn Marketing Program of Michigan. Samples should indicate the location where the sample was collected.

Source: MSU