Tar spot on corn leaf

Figure 1. Magnified image of tar spot on corn leaf. Black spots are slightly raised on the upper surface of the leaf and cannot be wiped or scratched off the leaf.

Updated: July 17, 2023
By Andrew Kness

Scout for Tar Spot in Corn

As many of you are aware, we confirmed tar spot for the first time in Maryland during the 2022 growing season. Tar spot is a foliar disease of corn caused by the fungus Phyllachora maydis.

This disease has the potential to significantly impact yield. Tar spot may appear in corn on any above-ground plant surface and may appear at any growth stage; however it is often not observed until late vegetative to reproductive stages. P. maydis requires cooler temperatures and moisture to infect corn. Over the past few weeks we have had relatively cool nighttime temperatures, coupled with rain (leaf wetness) and high humidity. This combination of cool temperatures and wet plants, along with low light intensity from prolonged cloud cover and smoke from wildfires to our north, could flare tar spot infections. I encourage you to scout your fields in the coming weeks to look for the raised, black lesions of tar spot (Figure 1).

Research has indicated that fungicide applications around VT/R1 have the best odds of return on investment for managing tar spot; however, in some cases a second application made as late as R4 may also provide an economic benefit if weather conditions remain cool and wet. As you can see in the Fungicide Efficacy Table for Corn; most fungicides do a good job of managing this disease; however, products with two and three-way modes of action tend to provide better control.

Tar spot can also cause corn to senesce and dry down very rapidly, so be aware of this if you are growing corn for silage as a tar spot infestation can take a field from above chopping moisture to well below optimum in just 10-14 days.

This year we were awarded a grant from the Maryland Grain Producer’s Utilization Board to evaluate fungicide timing for managing tar spot; data will be provided at the end of the season. A second part of this project is to get a handle on how prevalent tar spot is across Maryland. If you find tar spot in your fields, I would be interested in knowing about it. You can call, text, or email me (akness@umd.edu), or you can report a sighting at https://corn.ipmpipe.org/reporting-form/