The Ontario Ministry of Agriculture, Food and Rural Affairs released its Field Crop Report for September 15. In it, they discuss how areas experiencing several inches of rain, cool temperatures and high relative humidity have an increased risk of tar spot developing.
Tar spot is caused by a pathogen called Phyllachora maydis and can cause severe yield loss when conditions are favourable for disease. This pathogen overwinters on infested corn residue on the soil surface.
Tar spot was confirmed first seen in the Midwest US in 2015, believed to have blown in from Mexico. Since 2015, tart spot spread in Indiana, Ohio and particularly Michigan, which was conferred to have gone over the Ontario boarder in 2020.
Throughout the 2021 growing season, tar has expanded from the original lower five counties in Southwestern Ontario, to as far as Wellington county and North into Bruce and Grey counties over this past month.
Growers who are planting corn-on-corn this year, should be scouting early and often for tar spot.
Tar spot can be scattered across the upper and lower leaf surface as tiny, raised, spore-filled black spots. These are the first visual symptoms of tar spot to appear, although the plant may be infected long before symptoms are visible.
Untreated tar spot negatively affects the plant’s ability to photosynthesize, causing leaves to brown and die early.
To reduce tar spot development and severity, manage residue, utilize rotation and avoid hybrids.
Fungicides should be applied to reduce disease development; however, it only effectively manages disease for about 14 to 21 days after application. Applying a fungicide late in the season will not ‘cure’ an already diseased plant.
If you see tar spots on your crop, consult with your local agronomist for treatment options.