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).

A new and effective method to induce infection of Phyllachora maydis into corn for tar spot studies in controlled environments

“…This includes studies of factors influencing host-pathogen interactions and the ability to identify resistance traits for genetic improvement of corn, which is urgently needed for effective management. Additionally, the non-replicability of methods to induce tar spot and the lack of information about the disease have impeded the completion of Koch's postulates for the first reports of the disease since 2015 [1] to 2023 [16] in the U.S. and Canada. In this study, we developed an effective and consistent method to induce tar spot of corn in controlled conditions.…”

Uncovering the Infection Strategy ofPhyllachora maydisduring Maize Colonization: A Comprehensive Analysis

“…Phyllachora maydis is an obligate fungal pathogen that causes tar spot, a foliar disease of maize (Zea mays L.) common in the United States, Mexico, Central and South America (Bajet et al 1994;Mottaleb et al 2019;Rocco da Silva et al 2021;Solórzano et al 2023;Valle-Torres et al 2020). Tar spot was first detected in the U.S. in 2015 (Collins et al 2021;Malvick et al 2020;Mottaleb et al 2019;Moura et al 2023;Pandey et al 2022;Rocco da Silva et al 2021;Ruhl et al 2016;Valle-Torres et al 2020;Wise et al 2023). Since then, P. maydis has caused an estimated $2.9 billion in maize yield loss (Mueller et al 2021(Mueller et al , 2022.…”