Tweak tar spot plans based on weather

Tar spot hit hard in 2021. Last year we sprayed at V10 and after pollination. We didn’t see much tar spot. One untreated field yielded as good as treated fields. Can we spray only at pollination this year, and only if we see disease?
The Indiana certified crop adviser panel answering this question includes Gene Flaningam, agronomist with Flaningam Ag Consulting LLC, Vincennes; Troy Jenkins, agronomist with Ceres Solutions and 2022 CCA of the Year; Greg Kneubuhler, agronomist with G&K Concepts Inc., Harlan; and Dan Quinn, Purdue Extension corn specialist.
Flaningam: The environment must be just right for the disease. Field conditions that favor tar spot are prolonged periods of leaf wetness and cool temperatures, such as heavy fogs. Irrigated fields show higher risk. Return on investment for applying a fungicide will be determined by plant growth stage and disease pressure at the time you want to make the application. Corn near maturity will not be target fields that need fungicide. Scout later-planted fields instead.
Jenkins: We suspect tar spot infection was delayed in ’22 due to extreme heat and drought. Wet and cooler conditions allow inoculum to move from residue to lower parts of the corn canopy. Spores can blow higher into the plant, and infection becomes severe. We must scout for tar spot and be vigilant. Using the Tarspotter app and university predictions can help determine when to make first and subsequent fungicide applications. You might need to spray early if disease is seen, or you might be able to wait up to pollination, if disease is delayed.
Kneubuhler: Any plant disease needs to meet the disease triangle: environment, pathogen and host. The pathogen exists now in most counties. Missing from the triangle in ’22 was the right environment. Much of Indiana had a warm, dry May-June period, which didn’t allow tar spot to get a foothold. Those requirements were met late in the season and tar spot showed up, but yield was not impacted. That’s why you may have seen a field yield nearly as good as a treated field.
Initial data work done on tar spot suggests that a tassel time application of fungicide will be sufficient. Under extreme pressure like in ’21, a V10 application plus a tassel application pays big dividends. At a minimum, a tassel application should be part of your 2023 plan.
Quinn: The difficulty with tar spot is that it can become prevalent in corn at different times of the growing season depending on the year, due to weather conditions and rainfall. In 2022, dry conditions during June caused the disease to come in much later, and it did not cause much of a yield impact. Year in and year out, spraying fungicide at pollination is the most consistent timing. It is very rare that a vegetative application plus a pollination application is worthwhile, and only in severe cases when the disease comes in early.
Scout your fields, especially if you have a history of tar spot, and make an informed decision. Fungicide applications rarely have a response in years where disease is low or non-existent. In addition, follow the Purdue field crop pathology team.