Tar spot (Phyllachora maydis)

Tar spot: biology

Tar spot is caused by the fungus Phyllachora maydis. The fungus overwinters on infested corn residue on the soil surface. In fields with infested corn residue, initial signs and symptoms of tar spot may appear in the lower canopy of the corn plant. However, “top down” patterns of disease can occur in previously clean fields, suggesting the spores can travel significant distances via wind.

High humidity, cooler temperatures (16-21°C) and prolonged leaf wetness (7+ hours) favour disease development.

The disease has a long latent period of 14-21 days. Tar spot has multiple generations during one growing season, with each generation capable of producing inoculum. Rain and high humidity cause the release of spores which are dispersed by rain splash or wind.

Tar spot was first identified in Ontario corn fields in 2020 and 18 counties have reports of tar spot by the end of the 2021 growing season, according to the Corn ipmPipe website.

Tar spot symptoms include small, raised black spots called stromata on corn leaves and husks. Tan halos of necrotic leaf tissue around the black lesions, referred to as fisheyes lesions, can also occur.

Tar spot is often found during the mid to late grain stages (R3-R6) of corn.

Tar spot can look similar to rust when older rust spores turn dark brown or black. To differentiate between tar spot and rust, rub the lesions between your fingers. If the lesions come off of the leaf and smudge your finger, it is not tar spot.