Was Tar Spot a 'one and done' disease problem or here to stay?

Joe Lauer
UW Madison
Tar spot fungus intensified in Wisconsin corn in 2017. Hot spots in Southwest Wisconsin saw as much as a 20 percent drop in yields in 2018.

In 2018, southwest Wisconsin was especially hard hit with a new disease called Tar Spot, Phyllachora maydis. I have talked to many growers this past winter about the disease and what might be done for the coming season. However, we have limited experience with the disease and it's implications for yield.

At Montfort we had a significant tar spot infection in our hybrid trial plots. It was the only disease present. Dr. Damon Smith was able to rate each plot for the disease.

Later we combine harvested each plot measuring yield, moisture, lodging and test weight. Dr. Smith rated ear leaf disease severity of 45 to 50% which correlated to yield impacts of 40 to 60 bu/A (18 to 27%). This is a disease that needs to be reckoned with in the future.

Counties in the Midwest where tar spot infections were confirmed in 2019.

So how do we plan for 2019? Is tar spot a "one and done" disease, or is it here to stay? For any disease to be a problem, it needs a susceptible host, a virulent pathogen and favorable environmental conditions. All conditions have to be present. Since tar spot has affected yield during one growing season, the prudent thing to do is plan for the disease in the future.

To reduce tar spot development and severity, Kleezewski et al. (2019) recommends managing residue, crop rotation, using hybrid resistance, and using fungicide. Of these recommendations, crop rotation might be the easiest management tool to implement. Many fields in southwest Wisconsin are no-till planted so burying residue is problematic.

Using hybrid resistance might be effective, but little public information is available about hybrid/family disease reaction of commercial hybrids sold to farmers. Some fungicides may reduce tar spot, but there is little data about application timing that provides an effective and economical response.

More to come on this issue.

Joe Lauer

Joe Lauer is an agronomy professor at the College of Agricultural and Life Sciences at University of Wisconsin and is an expert on corn production, transgenic crops and cropping systems in the Midwest U.S..