Taking an environmentally sensitive approach to pest management


Mandy D. Bish
University of Missouri
Plant Science & Technology
(573) 882-9878

Tar spot of corn increases its footprint in Missouri

Mandy D. Bish
University of Missouri
(573) 882-9878

August 25,2023

minute read

map of Missouri counties with the northern countes colored with gold and grey

Figure 1 Current map of counties with confirmed tar spot. Gold = confirmation in 2023. Gray = confirmation in a previous year. Map generated at https://corn.ipmpipe.org/tarspot/

Tar spot was first confirmed in northeast Missouri in 2019, northwest Missouri in 2022, and many additional north central counties in 2023 (Figure 1).

Many of you have been involved in scouting and reporting efforts, and it is much appreciated as we are catching the disease very early in some instances.

Most tar spot incidences in Missouri prior to 2023 were confirmed in late August, early September, as the corn was passing from the dent or R5 growth stage to physiological maturity.

This year the disease showed up early but then we had extremely hot and dry conditions in July and disease progress seemed to slow. Conditions changed in late July and early August, and we began receiving additional reports.

Along with reports, we have received some questions about applying fungicide at the dent (R5) growth stage in fields where tar spot has come on rapidly. I am unaware of research that supports these late applications. On the contrary, research exists that indicates a lack of net return on the R5 application. Dr. Telenko's group at Purdue has led much of the initial research on tar spot. From 2019 to 2021, the group conducted 3 years of research on fungicide application timings in a field with a history of severe tar spot. In that study, the group noted the highest net returns occurred when applications were made from VT/R1 through R3. They observed some benefit to the R4 (dough stage) application as well. Check out Figure 4 in her publication to see the full range of timings studied.

What now? For this season, continue to scout and become familiar with this disease and where it is being found. The fungal pathogen that causes tar spot, Phyllacora maydis, can survive Missouri winters. Where the disease is a problem in 2023, we can anticipate it being a problem in future years. The disease can come on quite aggressively as some of our farmers in northwest and northeast Missouri have unfortunately learned this season. Knowing where the problem areas are will be helpful in guiding 2024 scouting and fungicide applications (and subsequent years).

Secondly, please submit diseased samples to the MU Plant Diagnostic Clinic for confirmation of tar spot, especially if you are unfamiliar with this disease. Photos can be submitted electronically. In some instances a physical sample can be necessary. Also, please consider reporting the disease along with images to: https://corn.ipmpipe.org/reporting-form/. We confirm those reports and update the IPMPipe map. This helps us all keep up-to-date with where tar spot has spread, and will assist in future management recommendations.

Third, discuss hybrid selection with your seed dealer before ordering corn for 2024. The disease has been in the US long enough that seed companies are starting to feel more confident in which varieties are more tolerant.

For tar spot identification tips, visit this previous Mizzou Crop & Pest newsletter or the Crop Protection Network resources, including an overview of the disease and the severity estimation tool.

Also, consider subscribing to the MU Crop & Pest Text Alert System (ipm.missouri.edu/croppest/textAlert.cfm) to keep up to date on the latest information about tar spot.

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REVISED: August 25, 2023