P-290 - Effect of planting date and fungicide application on the development of tar spot on corn

Tar spot is a foliar disease of corn (Zea mays L.) disease caused by the fungus Phyllachora maydis that may result in substantial yield loss. The objective of this study was to assess the effect of planting date and fungicide application on the development of tar spot in southeast Iowa in 2024. Corn was planted April 24 and May 20, and sprayed with a fungicide at silking (R1) or not sprayed. There were 6 replications. Tar spot severity on five leaves of each of five plants was assessed approximately every two weeks from 10 days before tasseling (VT) through physiological maturity (R6). The data were used to calculate area under disease progress curve (AUDPC) values for each leaf. Tar spot was first observed in each planting date at R1 and disease severity progressed through the season. Final disease severity of the ear leaf  reached 3.2 % and 12.9 % in the early and late planted treatments, respectively. No effect of fungicide treatment was detected on AUDPC of all leaves assessed (P >0.05), but an effect of planting date on AUDPC was detected for each leaf assessed (P< 0.01). AUDPC values for the late planted treatment were greater than those for the early planted treatment. These data suggest late planted corn is more at risk for tar spot, likely because conducive environmental conditions occur earlier in the grain fill period and consequently more severe disease develops.