In 2018, tar spot was detected in 12 counties in eastern Iowa. The disease was observed late in the growing season, at did not cause a lot of damage. In Wisconsin, Illinois, Michigan and Indiana, however, the disease was observed earlier during grain fill and caused significant yield loss.
Dr Damon Smith has developed a tar spot prediction model, Tarspotter, that is being validated this growing season by University extension and industry colleagues throughout the Midwest.
Figure 1 shows the calculated risk from Tarspotter for June 23, 2019 for a few locations in eastern Iowa. We have fungicide trials at the ISU Northeast Research Farm (NERF), near Nashua and the ISU Southeast Research Farm (SERF), near Crawfordsville. Both of these research farms do NOT have a history of tar spot. Waterloo, Dubuque and Davenport locations are situated in counties where tarspot was observed in 2018. Johnson is an ISU research farm in Central Iowa where tar spot has not been observed.
Figure 1 shows that the present risk for tar spot is high in northeast Iowa.
Should I spray a fungicide now on my corn in eastern Iowa?
No. Tar spot is yet to be observed in 2019 in Iowa, and even further east in Wisconsin, Illinois, Michigan and Indiana, although the weather has been conducive. Carefully scout fields with a history of tar spot that are at growth stage V6 or older. A fungicide application is only needed if the disease is observed.
If you observe tar spot, please contact me or your ISU Extension Crop Specialist. We would like to collect data from the field to contribute to Dr Smith's validation of Tarspotter.