Tar Spot Map 2025. Malvick provided a review of the corn tar spot disease biology and the spread of the disease in corn production fields this year across minnesota and into adjoining states. Yield loss isn’t typically detectable in the field until severity reaches 10% or more on the ear leaf or leaves above this leaf.
In this paper, we provide a brief description of current knowledge about tar spot of corn, including pathogen taxonomy, host range, symptoms and signs, specimen storage, pathogenicity testing, diagnostic protocols, and geographic distribution. We have received several calls related to fungicide application and, more specifically, tar spot identification and treatment.