Tar Spot Update

We have received several calls related to fungicide application and, more specifically, tar spot identification and treatment.

The map shows what counties have had a positive ID of tar spot in the past (gray), and what counties have positive identification of tar spot as of 7-31-23 (yellow). This tar spot map and maps showing progression of other diseases can be found at https://corn.ipmpipe.org/.

Tar spot map as of 7-31-23

Causes

Infection of any disease has three requirements as shown in the diagram.

1. The disease or pathogen must be present.
2. It must have a susceptible host.
3. The environment must be conducive for the pathogen to grow and thrive.

The pathogen or disease is present in our area as shown by the map. Tar spot can overwinter in corn fodder; the host is present as well, corn. The key to this is environment. Moderate temperatures, high humidity and leaf wetness are what drives tar spot infection. We have more moderate temperatures than most of the area for the next 10 days, and heavy dews have been prevalent as well. Phone apps such as Tar Spotter can help give you the likelihood of infection and give recommendations of when to spray a fungicide.

Identification

•  Small, black, circular, raised lesions called stromata appear on leaves, husks, and tassels.
• Stromata cannot be removed from leaf tissue with water or rubbing.
• Commonly misdiagnosed with frass (insect poop), rotten or aged pollen, rust pustules, these can be removed by rubbing the leaf or scraping with your fingernail.

Treatment

Fungicides containing both a Group 3, DMI Triazole, and a Group 11, Strobilurin, modes of action have been shown to provide the best suppression of tar spot. A comprehensive list of fungicides and their efficacy to tar spot, as well as other disease control, can be found at the Crop Protection Network website.  

The timing of fungicide in corn is of great debate. In the absence of disease, the tassel timing of fungicide has shown to provide the most return. The tassel timing is when 80% tassels are emerged and anthesis, or pollination, has not yet begun. If you miss this window, the next best fungicide timing is once the silks turn brown (R2-R3). When the silks turn brown, pollination is complete. Pre harvest interval or PHI is also something to be mindful of especially regarding silage or high moisture corn harvest. PHI is the amount of time that must pass between application of a pesticide and the harvest of the crop. Some fungicides have a PHI of up to 30 days. You can find information on fungicide PHI on the Crop Protection Network website mentioned previously in this article.

Brian Weller
Western Region
507.456.3034

Rod Moran
Western Region
507.456.3034

Dan Ritter
Central Region
219.863.0583

Branden Furseth
Northern Region
608.513.4265

Mark Gibson
Eastern Region
260.330.8968

Amanda Goffnett
Eastern Region
989.400.3793

Ryan Mueller
Eastern Region
989.400.3793

Enjoying our Agronomy Updates? Suggestions for topics you'd like us to weigh in on? Drop us an email at dairylandseed@dairylandseed.com. We'd love to hear from you!