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First Report of Tar Spot of Corn Caused by Phyllachora maydis in Minnesota

Tar spot, caused by Phyllachora maydis Maubl., can be a significant disease of corn (Zea mays L.). Commercial corn hybrids with typical signs and symptoms of tar spot were observed and collected in fields from Fillmore and Winona Counties, Minnesota, in late September and early October 2019. The disease in Fillmore County appeared to be focused in one area of the field about 10 m in diameter with incidence estimated at 90% and severity on individual leaves at and above the ear ranging from 1 to 34%. The field in Winona County had leaves with a similar level of severity on field edges. Signs and symptoms on the epidermis of leaves in both fields consisted of small black, raised, circular stroma (0.5 to 4 mm), primarily on the upper surfaces of leaves. All lesions were assessed, and less than 3% of tar spots were associated with fisheye lesions or chlorotic halos. Microscopic examination of the stroma from both locations revealed asci and oval to ovoid, hyaline ascospores of characteristic size and appearance for P. maydis as described by Parbery (1967). Specifically, 40 randomly selected stroma were dissected from the Fillmore County sample, and ascospores within had an average length of 9.8 μm and width of 6.6 μm. Identity of the stroma from samples in both counties was further confirmed by DNA sequencing. Genomic DNA was extracted from stroma, and a portion of the RNA gene (491 bases) was amplified by PCR using ITS4/ITS5 or ITS1F/ITS4 primers (White et al. 1990). The sequence similarity using BLASTN for all samples was 96 to 100% for four accessions of P. maydis (NCBI GenBank accessions MG881846.1, MG881847.1, KU184459.1, and MK184990.1). Due to the biotrophic, obligate nature of P. maydis, Koch’s postulates were not conducted (Parbery 1967). This confirms for the first time that tar spot of corn caused by P. maydis occurs in Minnesota. These results document an increased distribution of tar spot in the United States since it was first found in 2015 (Ruhl et al. 2016), which includes at least two counties in Minnesota, where it could cause significant economic losses for corn producers.