Tar Spot, caused by a fungus called Phyllachora maydis and native to Central America, is still a relatively new disease in the United States. The Crop Protection Network tracks Tar Spot's past and current distribution, and Beck's agronomists also keep tabs on the disease and effectiveness of fungicide applications on its suppression. Read below for a regional update on Tar Spot in the USA.

NORTHERN REGION

Tar Spot was present in many areas of Wisconsin again in 2024. In general, the western half of the state experienced the heaviest pressure, and yield losses occurred in the hardest-hit areas. With the extremely wet, cloudy, and cool June, I expected there to be even more Tar Spot throughout the state, but the much drier July weather suppressed disease development in the east. Fungicide applications were generally effective at slowing down Tar Spot, but the timing of the application played a huge role in how effective this practice was. A VT application in mid-July was too early in some areas, as the disease was able to develop by mid- August. However, some applications made in early- to mid-August at the late R2 (brown silk) to R3 (blister kernels) were likely too late, as a few black lesions were already visible on the corn leaves, and the disease still developed rapidly soon after the application. The ideal fungicide application timing for Tar Spot is less certain than many other corn foliar diseases, but an R1 (silking) to R2 (brown silk) target is looking like the best practice. Many newer hybrids have improved genetic tolerance to Tar Spot, and genetic tolerance continues to be the strongest management tool for Tar Spot.

By Joey Heneghan

Field Agronomist - Wisconsin

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WESTERN REGION

This was the second year of known infection of Tar Spot in Nebraska, and all 18 counties in the southeast portion of the state had confirmed infections. Most infections began late June through early July, but the hot, dry weather slowed down the infection's intensity in most fields. This year, there was a more widespread infection rate as most growers faced some level of infection in their fields. Hybrid sensitivity was also easier to identify due to the widespread infection. Last year, we educated growers on fungicide application timing and potential yield losses without fungicide applications. This year, many, if not all, irrigated fields were sprayed, and we saw reduced infections. Nebraska dryland fields experienced a negative ROI from fungicide applications the past two years, so there was hesitation to spray those acres this year. Dryland fields that were not sprayed and had more sensitive hybrids had significant yield losses (20-40 Bu./A.) and poorer plant health going into harvest. I believe this year will help growers understand the potential negative impacts that this disease can have. As we move forward, we will realize how important scouting and applying fungicide is to control and mitigate yield impacts.

By Trey Stephens

Field Agronomist - SE Nebraska

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EASTERN REGION

Tar Spot was first detected in mid- June, much earlier than prior years. Given the early infections, we prepared for a big Tar Spot year. For much of the late vegetative stages and into early reproduction, Tar Spot remained isolated to just a few areas. It wasn’t until early- to mid-August that Tar Spot progressed rapidly, often in R2 to R4 corn. Infections were noticeably worse in susceptible hybrids, when a fungicide was not applied, and when corn was deficient in key nutrients, especially nitrogen. Fungicide passes made after tassel and later during R2 and R3 made a big difference this year however, due to low commodity prices and the later onset of the disease, many fields were not sprayed. In heavily infected fields, we saw significant yield losses and poor stalk and grain quality where plants died prematurely. In future years, scouting for disease presence and severity will remain a critical step for Tar Spot management. Growers also must keep corn healthy through good nutrient management practices, and more will need to make fungicides a regular part of their management practices to effectively manage Tar Spot when we do have a bad infection year.

By Aaron Brooker - Ph.D

Field Agronomist - Michigan and N. Indiana

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CENTRAL REGION

While we had a conducive environment for Tar Spot around tassel time, we did not see a vast infestation of the disease until late August or early September. Our area was very dry in June, and it appears as though that may have inhibited some of the early development of the disease, leading to that late occurrence. While what we have seen late-season looks worrisome, I have yet to find many instances where Tar Spot will be limiting to yield in any major way. In some cases, this late outbreak did trigger growers to make a second application of fungicide around R3-R4, but most of those applications have not shown a yield increase. My biggest concern with this is that Tar Spot will overwinter in residue, and with many corn-after-corn operations moving to reduced tillage, we could see a larger amount of inoculum ready to infect those fields in 2025. Overall, we will need to continue to scout for Tar Spot before tassel and utilize hybrids with good disease tolerance in these situations.

By Drew Beckman

Field Agronomist - C. Illinois

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