Integrated Management of Tar Spot of Corn with Organically-Allowed Fungicides in the Midwest United States

Tar spot of corn, caused by the fungus Phyllachora maydis , has emerged as a major foliar disease that can cause significant yield loss across the Midwest. First identified in the continental U.S. in 2015, P. maydis rapidly spread and is now documented in 20 states and in Ontario, Canada. Currently, there is limited information available on integrated management strategies for organic grain production systems, including the effectiveness of host resistance in organic hybrids and the efficacy of Organic Materials Review Institute (OMRI)-listed foliar fungicides. To fill this gap, field studies were conducted in Indiana and Wisconsin during the 2021, 2022, and 2023 growing seasons. These studies evaluated two organic hybrids (tar spot resistant and susceptible) and the efficacy of OMRI-listed foliar fungicides for tar spot and yield protection. In both Indiana and Wisconsin, tar spot severity varied greatly among years, and hybrid resistance provided the most consistent reduction in tar spot severity, particularly under moderate to high disease pressure. Among the products evaluated, only the conventional fungicide pyraclostrobin + metconazole consistently reduced tar spot and protected yield. Among the OMRI- listed products evaluated, copper oxychloride + copper hydroxide, Streptomycese lydicus , and Bacillus amyloliquefaciens only reduced tar spot severity under specific site-years and hybrid combinations; however their performance was inconsistent across environments and did not consistently translate into yield protection. Based on results host resistance is the most reliable strategy for managing tar spot in organic grain production, while currently available OMRI-listed fungicides provide inconsistent tar spot suppression and yield protection.