According to the latest Crop Protection Network map, positive cases of tar spot have been reported in 26 Iowa counties so far this growing season. First reported in the Corn Belt in 2015, the disease has been confirmed in Iowa since 2016. Now all 99 Iowa counties have a history with the disease. This season, tar spot has also been confirmed in neighboring Wisconsin, Illinois, Missouri, and Nebraska. Kansas, Indiana, Michigan, Pennsylvania, and New York have reported positive cases as well.
Gentry Sorenson covers a number of northwest Iowa counties as an extension agronomist. He says the crop is late stage vegetative to R2 in his area. That’s about the right time to consider applying fungicide to target tar spot. Multi-state fungicide research results show that applications made for tar spot management are most effective and economical when made between R2-R3, even when disease developed during early vegetative stages.
-NAFB