by Alison Roberston
Tar spot has arrived in Iowa, with the confirmation of tar spot in two counties: Poweshiek County and Audubon County. Its presence also is probable in Guthrie County, according to a Crop Protection Network tar spot map.
This is no surprise, considering when tar spot initially surfaced in previous years in central Iowa:
- June 30, 2022
- June 23, 2023
- June 7, 2024
The triangle rules!
The disease triangle applies in tar spot infestations. For disease to occur, it needs to have a:
- pathogen
- host
- favorable environment
Every county in Iowa has a history of tar spot, so the disease is present. Every Iowa county also grows corn, so the host is present.
What really matters when it comes to tar spot, however, is the weather, especially temperature. Mean daily temperatures of 64-73 degrees F for 30 days and relative humidity of less than 90% for two to three weeks favor the disease. Doesn’t that sound like typical spring conditions in Iowa?
Just because tar spot is surfacing now, though, doesn’t mean there will be widespread tar spot epidemics in July and August. Temperatures greater than 73 degrees F 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 78 degrees F, which considerably slows development of the disease.
The past four years have reiterated that observations of tar spot in June do not lead to widespread epidemics in July and August, likely because it was just too hot for the disease to develop. Indeed, when I looked at 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 fungicide applications
For now, scout, but hold off applying a fungicide until VT (tassel) through R3 (kernel milk stage).
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 (tassel) to R3 (kernel milk stage) have been the best timing to manage the disease.
If growers spray a fungicide now (V6-V10), they will definitely need a second application later. They only will protect plant leaves, and those leaves contribute little to yield.
Also, the fungicide will have worn off by VT and R2 (kernel blister stage). A fungicide application applied between VT and R3 will protect a plant as it makes grain from R2 through R6 (physiological maturity).
Useful resources
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
- mapping where tar spot has been observed in the U.S.
Robertson is an Iowa State University Extension plant pathologist.