First report of tar spot on corn caused by phyllachora maydis in the United States

In early September 2015, the Purdue Plant and Pest Diagnostic Lab (PPDL) received leaves of hybrid corn (Zea mays L.) from Carroll and Cass counties, Indiana, and Bureau, Dekalb, and LaSalle counties, Illinois, showing small (0.5 to 1.5 mm), black, circular, raised structures on the epidermis of leaves. The black structures were observed on both healthy and necrotic tissue and were surrounded by narrow tan halos. Disease severity varied across samples from a variety of commercial hybrids, ranging from 1 to 80% of leaf tissue affected. Microscopic observation revealed single and clustered ascomata within a clypeus, containing cylindrical unitunicate asci with ellipsoid, hyaline, uniserate, aseptate ascospores 6 to 8.5 × 11 to 14 µm (n = 25), and abundant filiform paraphyses. Based on these characters, the fungus was identified as Phyllachora maydis Maubl., cause of tar spot (Parbery 1967). Samples were sent to the USDA-APHIS in Beltsville, MD, for official confirmation, since the disease had not previously been reported in the United States. The fungus on samples from Indiana and Illinois was identified morphologically and officially confirmed on 9 September and 18 September 2015, respectively. These samples have been accessioned at the U.S. National Fungus Collections (BPI) in Beltsville, MD. DNA was extracted directly from ascomata on leaves using a Qiagen Plant Mini kit (Qiagen, Valencia, CA). Amplicons of the internal transcribed spacer regions were generated using standard methods with primers ITS4 and ITS5 for four samples of P. maydis; two from Indiana and two from Illinois. The sequence of the ITS region from the Carroll County sample (BPI 893231) was submitted to GenBank (Accession No. KU184459). Phyllachora maydis is classified as an obligate biotroph and is considered to be nonculturable (Muller and Samuels 1984). Multiple attempts to culture the fungus were unsuccessful. No P. maydis accessions previously existed in GenBank, and sincePhyllachora spp. have primarily been identified by morphology, very few sequences of any species on any host exist in GenBank for comparison. To ensure the submitted P. maydis sequence is indeed a representative sequence of this fungus, it was compared and determined to be identical across the length sequenced to the sequence from a completely different P. maydis sample. After the initial confirmation, the disease was reported in five additional counties in Indiana, and 10 additional counties in Illinois. Tar spot is prevalent in Mexico and areas of Central and South America (Hock et al. 1992). Phyllachora maydis alone is not known to cause economic damage in these areas, but the presence of P. maydiswith another fungus, Monographella maydis, can result in severe disease (Hock et al. 1995).Monographella maydis was not detected morphologically in any of the U.S. samples, and no associated yield loss was observed in fields in Indiana and Illinois with the disease. The economic importance of the disease in the United States is still unknown at this time since symptoms were observed too late in the season to affect yield. Phyllachora maydis is not known to infect seed. It is suspected that inoculum may have been transported to the United States via weather events originating in Central America or Mexico.