Late Summer Tar Spot Update 2023

Tar spot has been the top yield-limiting corn disease in the upper Midwest the past few seasons. 2022 losses were estimated near 116 million bushels, about $14 per acre (according to Crop Protection Network). This year, the disease may not be hitting the Midwest quite as hard, but local severity can still be extreme; certainly, leading to losses greater than $14 per acre.

Figure 1. August 29, 2023, Tar spot distribution. https://corn.ipmpipe.org/tarspot/

The Dairyland Seed Agronomy Team agrees that the management window is quickly closing, if not already closed. We generally do not recommend fungicide applications after the early dent stage. The cost effectiveness of such applications becomes difficult to justify, and the risks of slower drydown increases.

Estimating the impact of infection becomes the next question. Tar spot is a dynamic disease that varies with weather conditions, hybrid resistance and hybrid maturity, so these are very general guidelines. Much of this information comes from multi-university trials that can be found on the Crop Protection Network website. To key in on the impact of the disease, we are going to focus on the severity of infection of the ear leaf at the kernel dent stage. Specifically, look for 30% and 50% severity as reference points. Photos in Figure 2 provide examples. Check out the Crop Protection Network Severity Estimation Tool for more examples and to quiz yourself.

Figure 2. Tar spot impact on corn based on disease severity at the R5 (dent) growth stage.