Phyllachora maydis is a plant pathogen causing ascomycete diseases in maize/corn, and is more commonly referred to as tar spot.

However, the accompanying fungal infection of Monographella maydis, identified by "fish-eye" lesions, was claimed to cause significant foliar damage and subsequently yield reduction.

Given the polycyclic nature of this pathogen, as well as the ability to infect corn at any developmental stage, it is extremely hard to manage.

[9] Native to the Americas, from Mexico south, P. maydis appeared in the US for the first time in 2015 in Illinois and Indiana, then Florida, Iowa, Michigan, Ohio, Minnesota, Missouri, and Wisconsin in 2019.

[9] A biological control method that has shown potential is the infection with Coniothyrium phyllochorae by reducing lesion size due to hyperparasitism.

[11][12] Chemical control with one or two treatments of Fenpropimorph or Mancozeb applied every ten days were the most effective fungicides used in field trials.

[8] Although no cultivars currently exist that are immune to this pathogen, CIMMYT has developed 14 inbred lines in Latin America that are highly resistant.

Tar spot causes low ear weight, vivipary, and poor kernel fill resulting in up to a 30 bushel loss per acre.