Tar spot is spreading to more areas of Michigan, with recent detection in Clinton and Huron counties bringing the number of counties with confirmed cases this year to nine.
The counties join Calhoun, Gratiot, Isabella, Jackson, Montcalm, Saginaw and Tuscola, where tar spot has been detected since the state's first confirmation of the year on July 13, according to the Crop Protection Network.
Beyond Michigan, tar spot has been detected in Canada and 13 other states this year, including Georgia, Indiana, Illinois, Iowa, Kansas, Kentucky, Minnesota, Missouri, Nebraska, New York, Ohio, Virginia and Wisconsin.
Michigan State University Extension recommends corn growers continue scouting fields for tar spot by looking for small, dark lesions on leaves. Growers can add water and attempt to rub off suspected lesions. If the lesion can be rubbed off, it is not tar spot but likely insect droppings, according to MSU Extension specialists Nicolle Ritchie and Angie Gradiz.
Fungicide applications for tar spot are generally most profitable when made between the R1, or silking, and R3, or milky kernel, growth stages in corn, the specialists said.
The Crop Risk Tool showed low to moderate risk for tar spot development during the past week. Cooler temperatures and irrigation can increase the risk of tar spot development.
According to the Crop Protection Network, research has shown a complex relationship between moisture and disease development. Prolonged periods of high humidity or excessive rainfall can be negatively associated with disease development or spore release.
Intermittent wet and dry periods also appear to influence tar spot development.
Source: Michigan Farm News, "Tar spot spreads to more counties: Here's what MSU Extension recommends for corn growers "