Phyllachora maydis, Causal Agent of Tar Spot on Corn, Can Overwinter in Northern Illinois

Tar spot on corn (Zea mays L.), caused by the obligate fungal pathogen Phyllachora maydis Maubl., was first detected in the United States in 2015. Currently, the disease has been detected in 172 counties across Florida, Illinois, Indiana, Iowa, Michigan, Ohio, and Wisconsin. Although observations indicate that P. maydis likely overwinters in the region, this has not been conclusively proven. Samples of corn foliage heavily infected with P. maydis were recovered from two fields in northern Illinois in March 2019. Ascospores were extracted and were applied to corn seedlings under controlled greenhouse conditions. Symptoms of tar spot were observed 17 days after inoculation, and ascospores were extracted from stromata to confirm P. maydis. This is the first conclusive proof that P. maydis can overwinter the United States. We also present a preliminary greenhouse method that, if optimized, may be used to study this pathosystem under controlled conditions.