Tar Spot
(Phyllachora maydis)

Tar Spot 2

Symptoms:  Small, raised black lesion on leaves, stalk, and husks that have a slight shine to them (looks as though someone flicked tar onto the leaf, hence the name). Disease reduces photosynthetic capacity and sever infection can reduce corn stalk integrity causing lodging.
Risk Factors: Overwinters in corn residue and is splash- or wind-dispersed to new corn plants. Disease is favored by cool (60-70° F), high humidity (>75% relative humidity), and prolonged leaf wetness (>7 hours).
Management: Proper identification of this new disease is very important, tools such as the Tar Spot Distribution map and the Tar Spotter app are useful to track tar spot each season. Practice rotation and tillage, where able, on infect debris. Plant resistant hybrids and when practical, fungicides may be applied (fungicide efficacy table).

For corn fungicide efficacy table, please refer to https://extension.tennessee.edu/publications/Documents/W342.pdf