Tar spot, caused by Phyllachora maydis, is the most significant yield-limiting disease of corn (Zea mays L.) in Indiana. Currently, fungicides are an effective management tool for this disease, and partial returns from their use under different disease severity conditions has not previously been studied. Between 2019 and 2021, two separate field experiments were conducted in each year in Indiana to assess the efficacy of nine foliar fungicide products and nine fungicide application timings based on corn growth stages on tar spot symptoms and stromata, canopy greenness, yield, and influence on partial returns. All fungicides evaluated significantly suppressed tar spot development in the canopy and increased canopy greenness over the nontreated control. Additionally, applications of mefentrifluconazole + pyraclostrobin, metconazole + pyraclostrobin, cyproconazole + picoxystrobin at tassel, and propiconazole + benzovindiflupyr + azoxystrobin between the tassel and dough growth stages were the most effective at significantly reducing disease severity, increasing canopy greenness, protecting yield, and offered the greatest partial return. Fungicide products varied in their ability to protect yield under low and high disease severity conditions relative to the nontreated control. Consistently, positive yield increases were observed when disease severity was high, which translated to greater profitability relative to low severity conditions. On average, the yield increases across foliar fungicide products and timed application treatments were 544.6 and 1,020.7 kg/ha greater, and partial returns using a grain value of $0.17/kg were $92.6/ha and $173.5/ha greater, respectively, when high severity conditions occurred. This research demonstrates that foliar fungicides and appropriately timed fungicide applications can profitably be used to manage tar spot in Indiana under high disease severity conditions.
Keywords: disease management; economic return; fungicide efficacy; fungicide timing; return-on-investment.