First Report of Tar Spot on Corn Caused by Phyllachora maydis in the Great Plains.

ABSTRACT

Tar spot caused by the fungus Phyllachora maydis Maubl. is a significant foliar disease of corn (Zea mays L.). Threatening corn production across the Americas, this disease can reduce the quality of silage and grain yield (Rocco da Silva et al. 2021; Valle-Torres et al. 2020). Lesions caused by P. maydis are usually black, glossy, and raised stromata on the leaf surface and occasionally on the husk. (Liu 1973; Rocco da Silva et al. 2021). Samples consistent with tar spot of corn were collected between September and October of 2022 from 6 fields in Kansas, 23 in Nebraska, and 6 in South Dakota. One sample was selected from each of the three states for further microscopic evaluation and molecular analysis. Signs of the fungus were visually and microscopically confirmed in eight Nebraska counties in October 2021; however, in the 2021, season tar spot sings were not found in Kansas and South Dakota. In the 2022 season disease severity varied by location; some fields in Kansas had <1% incidence, whereas in South Dakota disease incidence approached 1-2%, and in Nebraska between <1-5%. Stromata were present on both green and senescing tissues. Morphological characteristics of the pathogen were similar and consistent with the description of P. maydis (Parbery 1967) from all examined leaves across all locations. Asexual spores (conidia) were produced in pycnidial fruiting bodies ranging in size 1.29 to 2.82 x 8.84 to 16.95 µm (n = 40, average 1.98 × 13.30 µm). The pycnidial fruiting bodies were often found adjacent to perithecia within the stromata. For molecular confirmation, stromata were aseptically removed from leaves collected at each location and DNA extracted using a phenol chloroform method. The internal transcribed spacer (ITS) regions of the ribosomal RNA gene were sequenced using ITS1/ITS4 universal primers (Larena et al. 1999). Amplicons were Sanger sequenced (Genewiz, Inc., South Plainfield, NJ), and a consensus sequence for each sample was deposited in GenBank Kansas (OQ200487), Nebraska (OQ200488), and South Dakota (OQ200489). Using the BLASTn, sequences from Kansas, Nebraska and South Dakota showed 100% homology with 100% query cover to other P. maydis GenBank accessions (MG881848.1; OL342916.1; OL342915.1). Koch's postulates were not performed given the obligate nature of the pathogen (Muller and Samuels 1984). This report documents the first confirmation of tar spot on corn in Kansas, Nebraska, and South Dakota (Great Plains).