Tar spot has been found in Indiana, Michigan, and other parts of the Midwest already this growing season, so what does this mean for Ohio growers?
Now is the time though to monitor fields and make informed management decisions. Corn planting dates were highly variable this year and current growth stages range from anywhere between V2 and R1. Tar spot was observed this past week in the "Battle for the Belt" research plots at the Western Agricultural Research Station in South Charleston, Ohio. The corn was planted on March 25 and was at R1 (silking) growth stage. Tar spot incidence and severity on the ear leaf was low. For more information, please see this week's Battle for the Belt Update by clicking here.
When scouting for tar spot, prioritize earlier planted fields with a history of tar spot, and especially those that are irrigated as this can increase disease risk. Focus first on the lower canopy and look for small, raised black spots called stromata. Early in the season, insect frass can easily be confused with tar spot stromata. If the spot can be scratched or rubbed off with a wetted finger than it is not tar spot.
We also recommend using the Tarspotter disease forecasting app to determine if weather has been favorable for the development of the tar spot fungus. Moderate temperatures (64-73˚F), relative humidity greater than 75%, and short periods of leaf wetness are required for infection.
Fungicide applied between tasseling (VT) and R3 (milk) when disease is active has the most consistent return on investment based on research in Ohio and Indiana. Products with multiple modes of action are most efficacious against tar spot as well. You can find a fungicide efficacy table for corn diseases at https://cropprotectionnetwork.org/publications/fungicide-efficacy-for-control-of-corn-diseases.
If you suspect tar spot, please send an image or sample to the OSU Plant and Pest Diagnostic Clinic (ppdc@osu.edu). Leaf samples should be placed in a zip-seal bag as soon as it is collected and mailed to the address below using next-day or two-shipping option. If hand-delivering sample, please coordinate with Dr. Rotondo at 330-263-3721 or rotondo.11@osu.edu ahead of time.
OSU Plant and Pest Diagnostic Clinic
ATTN: Francesca Rotondo Department of Plant Pathology
Selby Hall – Room 234
1680 Madison Avenue
Wooster, OH, 44691