Tar spot was observed in Poweshiek county on 13 June 2025. This is no surprise! In previous years, tar spot was observed on 30 June 2022, 23 June 2023, and 7 June 2024 in Central Iowa.
The triangle rules!
Every county in Iowa has a history of tar spot and consequently the pathogen is present. Every county in Iowa grows corn, and consequently the host is present. What really matters however, when it comes to tar spot is the weather, especially temperature. Mean daily temperatures of 64-73F for 30 days, and relative humidity of less than 90% for 2 to 3 weeks, favor the disease. Doesn’t that sound like typical spring conditions in Iowa?
Meanwhile, temperatures greater than 73F and extended periods of greater than 90% humidity are not favorable for tar spot development. In Iowa, the mean daily temperatures in July and August are 76 to 78F, which considerably slows development of the disease. The past four years have reiterated that observations of tar spot in June do not lead to wide spread epidemics in July and August, likely because it was just too hot for the disease to develop. Indeed, when I looked at the the Crop Risk Tool for my field trial locations across Iowa, the risk of tar spot at each location declines considerably over the next week as daily temperatures increase.
Don't rush in when it comes to fungicide applications
Despite reports of tar spot in Iowa and the Midwest in June the past four years, an application of a fungicide is usually unwarranted when corn is still in vegetative growth stages. Research from across the Midwest and Canada has consistently shown that fungicide applications at VT to R3 have been the best timing to manage the disease.
Useful Resources
Remember the Crop Protection Network has several resources to help with fungicide decisions including:
- Choosing the best product for disease control
Fungicide efficacy for Control of Corn Foliar Disease
- Calculating your potential return on investment for a fungicide application
Determining if your field is at risk for tar spot or gray leaf spot
Map of where tar spot has been observed in the U.S.
Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Integrated Crop Management News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on June 16, 2025. The information contained within may not be the most current and accurate depending on when it is accessed.
- Professor of Plant Pathology and Microbiology
- Extension Field Crops Pathologist