An Observation of Corn Tar Spot Dispersal from Agricultural Fields to an Isolated Urban Plot

Tar spot on corn (Zea mays L.), caused by the obligate fungal pathogen Phyllachora maydis Maubl., was first detected in the United States in 2015 (Ruhl et al. 2016) and is currently present in over 310 counties across nine U.S. states (Kleczewski et al. 2020) and Canada (A. Tenuta, personal communication). P. maydis is known to only infect corn, and it produces diagnostic raised, black oval to elongated stroma embedded within the epidermis of leaves, stalks, and husks approximately 14 to 21 days after infection (Valle-Torres et al. 2020). Multiple perithecia are contained within a single stroma, and therein numerous ascospores are produced, which serve as the infectious unit for the disease (Valle- Torres et al. 2020). Very little is known about the biology of spore dispersal in this pathosystem and the airborne distance spores can travel. One controlled study was conducted in Mexico, where it was shown that P. maydis could travel as far as 75 m downwind (Hock et al. 1995). However, since 2018, plant pathologists in the Midwest have observed the disease in fields with no previous history of tar spot or from fields that were planted to soybean for one or more years prior to observation of disease (Telenko et al. 2019). Moreover, the disease was observed in the upper canopy and progressed down the plants, suggesting spores of P. maydis had been deposited from a distal area. These observations indicate a potential for P. maydis to spread considerable distances from an inoculum source. It is possible that airborne dispersal of P. maydis may be more important in parts of the Midwestern United States, a region dominated by flat topography containing millions of corn acres, as opposed to previous studies (Hock et al. 1995; Liu 1973). Consequently, controlled research focused on assessing movement of the pathogen in the Midwest is confounded by the immense acreage of corn and potential inoculum sources in the region. © 2021. The American Phytopathological Society