Corn and soybean diseases to watch out for in MN, including tar spot, were the topic of the March 5 “Strategic Farming: Let’s Talk Crops!” program hosted by University of Minnesota Extension. Dr. Dean Malvick, Extension Plant Pathologist with UMN Extension, joined moderator Claire LaCanne, UMN Extension Educator Crops, for a discussion on “Corn and Soybean Diseases – Looking back as we look forward”. This was the ninth weekly episode of the 2025 Strategic Farming: Let’s talk crops! webinars. The series runs through the end of March.
Tar spot was first discovered in the U.S. in 2015 and in MN in 2019. Since its initial discovery in MN, it has been spreading, causing yield losses in some fields, particularly in SE MN, that have exceeded 50 bu/ac. Tar spot overwinters in MN on corn residue, and the inoculum is currently present in many fields across the state, although the disease has not been a yield limiting factor in most areas. Wind borne spores and infected corn residue are sources of the disease inoculum.
The most severe and the only yield-limiting levels of tar spot to date have occurred in SE MN. Thus, this area is at higher risk from this disease than others so far, although there has been a pattern of inconsistency where tar spot severity can vary greatly in adjacent counties. Having a significant level of disease the previous year also does not necessarily mean there will be an issue the following year. Much research has been going into developing risk models for the disease. Although it appears the risk is low for the disease to develop across a wide area of the state in a given year, moderate temperatures (60 to 74°F), and an average relative humidity (more than 70% but less than 90%) seem to favor the disease.
The initial signs of the disease are black spots on corn leaves that will not rub off. As the disease progresses, infected leaves will eventually turn brown and die. When the disease develops early in the season, if conditions are conducive, the disease can complete several cycles in the field. About 18 days to three weeks are needed from the time plants are infected before the disease signs and symptoms appear. If the first signs of tar spot don’t develop until early August, there may not be enough time left in the growing season for the disease to be a significant yield limiting factor.
A fungicide application applied around VT (tasseling) and R1 (silking) up to the R3 (blister) stage of corn has been shown to decrease disease incidence when the disease is present in the field. If the disease develops early in the season, particularly when conditions are conducive for disease development, the likelihood of seeing a return to application increases. The Crop Protection Network (https://cropprotectionnetwork.org/) using research results from universities throughout the Upper Midwest, has a fungicide efficacy table for tar spot. Planting a tolerant hybrid is also a useful management strategy to reduce tar spot. Extending the crop rotation or tillage appears to provide little benefit For further details on tar spot and other diseases, check out the U of MN Extension crops website at z.umn.edu/crops and look under corn and soybean pest management. To watch this and other episodes: z.umn.edu/StrategicFarmingRecordings.
