As reported last week in Minnesota Crop News, tar spot of corn was found and confirmed for the first time in Minnesota in late September in southern Fillmore County. This find raised the question of whether tar spot also occurs in other areas in Minnesota. Now we know that tar spot has developed in multiple field and counties in Minnesota.
As of October 11, tar spot was confirmed in three additional counties. It has been found in two fields in Winona County, one field each in Faribault and Freeborn Counties, and in one additional field in Fillmore County.
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This fungal disease was at low levels overall in all of those fields, with no measurable yield loss expected due to the disease. These finds of tar spot in more areas emphasize the importance of scouting for this disease, both this year and next, so we know where the disease risk is highest and can take timely steps to manage it if needed.
The low levels of tar spot in Minnesota fields seem to match a similar pattern seen in many other parts of the Midwest this year. This disease is present numerous fields, but severity is low and yield loss due to disease is not expected.
Why did it stay at low levels in many areas and not develop to yield-damaging levels as happened in some other states in 2018? We suspect that part of the reason is that the requirements for disease development (high moisture/humidity, cool temperatures (60-70°F), and fungal inoculum availability) did not come together at the right time for infection to start early enough to cause serious plant damage.