Dichotomous Key for Identifying Corn Leaf Spot Disorders
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Collapse ▲- 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.