By Andi Anderson
Tar spot, caused by the fungus Phyllachora maydis, continues to affect corn fields across many regions. The disease thrives on infected residue, spreads via windborne spores, and can only infect green corn tissue.
While crop rotation and tillage offer little long-term protection, hybrid selection and timely disease management remain the best defenses.
Hybrid tolerance is the cornerstone of control. No variety is immune, but partial resistance can slow disease spread and may reduce the need for fungicide.
Farmers should consult seed dealers for disease ratings, particularly for tar spot, northern leaf blight, and gray leaf spot.
Scouting is essential, especially from tasseling (VT/R1) through milk stage (R3), when a single fungicide application often provides adequate suppression.
Occasionally, a later application at dough stage (R4) can help, but early vegetative sprays have little benefit. Two applications may reduce disease further, but profitability varies.
Fungicides with mixed modes of action are preferred to slow resistance development in P. maydis. Michigan State University Extension publishes annual fungicide efficacy charts to guide product choice.
Tar spot lesions are small, matte black raised spots that do not rub off, often surrounded by a thin brown margin. They can be mistaken for insect frass, rust pustules, or other leaf diseases, so correct identification is key.
Forecasting tools, such as the Crop Disease Forecasting platform, can help assess risk based on weather conditions. However, these tools complement rather than replace field scouting.
Economic return on fungicide use varies. Farmers are encouraged to leave untreated check strips and use ROI calculators from the Crop Protection Network to evaluate results.
Ultimately, integrated management—combining hybrid resistance, scouting, forecasting tools, and well-timed fungicides—offers the best chance of minimizing tar spot damage and protecting yields.
Photo Credit: gettyimages-dszc
Categories: Michigan, Crops, Corn, Rural Lifestyle, Farm Safety