- Are the spots circular with tan centers and purple to dark brown margins, possibly surrounded by yellow halos? → Yes → Go to 2 → No → Go to 4
- Are the lesions small, numerous, evenly distributed across the leaf, with a dry appearance and no water-soaked margin? → Yes → Eyespot (Fungus: Kabatiella zeae) → No → Go to 3
- Are the lesions larger, fewer, sometimes water-soaked, and irregularly distributed across the field? → Yes → Holcus Leaf Spot (Bacteria: Pseudomonas syringae pv. syringae) → No → Possible herbicide injury (e.g., paraquat drift). Look for uniform damage on similarly aged leaves and potential symptoms in nearby weeds or along field borders.
- Are the lesions long and narrow (cigar-shaped), gray-green to tan, and aligned with leaf veins? → Yes → Go to 5 → No → Go to 6
- Are the lesions 1–6 inches long, not limited by veins, and often gray-green to tan? → Yes → Northern Corn Leaf Blight (NCLB) (Fungus: Exserohilum turcicum) → No → Southern Corn Leaf Blight (SCLB) (Fungus: Bipolaris maydis) - Smaller, elliptical to rectangular tan lesions with reddish-brown margins)
- Are the lesions rectangular, with straight, parallel sides, tan in color, and bound by veins? → Yes → Gray Leaf Spot (Fungus: Cercospora zeae-maydis) → No → Go to 7
- Are there raised, shiny, black specks (stroma) resembling tar in or near the lesions? → Yes → Check if the black structures cannot be wiped off: • If firmly attached and embedded → Tar Spot (Fungus: Phyllachora maydis) • If easily wiped off → Likely insect frass • If dark areas are within tissue and not raised → Possible Pollen rot → No → Go to 8
- Are there rust-colored pustules on the leaf surface? → Yes → Go to 9 → No → Unknown cause – consider additional diagnostics
- Are the pustules dark reddish-brown, scattered on both upper and lower surfaces? → Yes → Common Rust (Fungus: Puccinia sorghi) → No → Southern Rust (Fungus: Puccinia polysora) - Lighter orange pustules, mostly on upper surface, often clustered → Unsure → See resource from Iowa State University
This dichotomous key is intended as a helpful tool to guide users in identifying potential issues and narrowing down possibilities. It is not a substitute for professional diagnosis or expert consultation. Always consult diagnostic resources or a qualified professional for accurate identification and management recommendations. For diagnostic support, submit a sample to NCSU's Plant Disease and Insect Clinic.
