First report of Phyllachora maydis causing tar spot on corn in Delaware

In October 2023, lesions consistent with descriptions of tar spot (Phyllachora maydis) were observed on corn (Zea mays) in Kent and Sussex County, Delaware (DE). Black, raised stromata were observed on leaves of commercially grown corn hybrids. Plants were at physiological maturity and disease severity was low with symptoms present on 1 to 10% of plants. In collected tissue, individual leaf severities ranged from 1 to 3% of leaf area with lesions. Hyaline conidia measuring approximately 15.5 µm in length and 0.5 µm in width were observed microscopically (n=5). Stromata were excised and sterilized in a 0.825% sodium hypochlorite solution for 30 s, rinsed in sterile deionized water for 30 s, and dried on a sterile paper towel for 30 s. Tissues were ground in a 1.5 mL microcentrifuge tube with a sterile plastic pestle. DNA was extracted using a DNeasy Plant Mini Kit (Qiagen). DNA was amplified at the internal transcribed spacer (ITS) region with ITS4 and ITS5 primers using polymerase chain reaction (PCR). NCBI BLAST search results yielded 100% sequence homology and 100% query cover (350/515 bp) to P. maydis accession MG881848.1 (Moura et al. 2023). Koch's postulates could not be completed due to the obligate nature of P. maydis. Tarspot was initially discovered in the United States in 2016 in Indiana and Illinois (Ruhl et al. 2016).This is the first report of tar spot on corn in DE. Yield losses from P. maydis can range depending on time of infection, environmental factors, and hybrid susceptibility and have been recorded up to 100% (Rocco da Silva et al. 2021). Because the disease did not enter the area until the end of the season, no yield impact was observed for 2023. Monitoring for the progression of disease will be crucial for future seasons (Telenko et al. 2020). High humidity and moisture levels favor disease development. Approximately half of DE corn acreage is irrigated due to sandy soils, current irrigation timing strategies may need to be reevaluated. Fungicide efficacy trials for management of tar spot have been conducted in other regions, but continued research will be needed to assess management options and optimize application timing for farmers in DE and the Mid-Atlantic region.