Gray leaf spot on corn — symptoms and treatment

Severity: medium

What is Gray leaf spot

Gray leaf spot is caused by Cercospora zeae-maydis. It's one of the most significant foliar diseases of corn. The fungus destroys leaf tissue, and because corn fills its grain using the leaves above and around the ear, losing that tissue during grain fill is what turns a leaf disease into a yield problem. Severity depends heavily on how much infected residue is left on the surface and how humid the season turns.

Symptoms

The lesions are the giveaway, and their shape is unusually distinctive. They are rectangular — blunt-ended, gray to tan, with straight parallel sides — because the fungus is stopped by the leaf veins and can only grow along the channels between them. That is why the lesions run parallel to the leaf veins, giving them a ruler-drawn, almost machined look that few other corn diseases produce. As infection builds, lesions may coalesce, and enough merging lesions blight the whole leaf.

• Early: small lesions on the lower leaves, developing a rectangular outline
• Early: gray to tan colour, sides bounded straight by the veins
• Advanced: lesions coalesce into large dead areas
• Advanced: leaf blight; loss of green tissue during grain fill

Compare with Northern leaf blight, whose lesions are long, cigar-shaped and gray-green and are not boxed in by the veins. If the lesion has straight parallel sides and blunt ends, it is gray leaf spot.

Causes and conditions

Cercospora zeae-maydis survives on corn residue left on the soil surface. That residue is where the next season's epidemic starts: spores are produced on it and carried by wind and splashing rain up onto the lower leaves, and the disease then works its way up the plant leaf by leaf. Warm, humid conditions with extended leaf wetness — heavy dew, fog, humid nights — favour infection. This is why continuous corn under reduced tillage is the classic high-risk situation: the inoculum from last year is sitting on the surface, right underneath this year's crop. Dense canopies that hold humidity make it worse.

Treatment

Crop rotation and tillage — cultural

Timing: post-harvest and planting. Rotate with non-host crops. Till to bury crop residue which harbors the fungus.

Azoxystrobin (Amistar) — chemical

Timing: early tassel stage. Apply foliar fungicide at early tassel stage if disease pressure is high. Pre-harvest interval: 21 days.

Pesticide registrations vary by country — check local approval before use.

Prevention

• Rotate away from corn to a non-host crop so surface residue is no longer feeding the disease.
• Bury crop residue with tillage where rotation alone is not enough.
• Choose hybrids with better gray leaf spot tolerance for fields with a history of it.
• Avoid excessively dense stands that trap humidity in the canopy.
• Scout the lower leaves before tassel — that is where the decision to spray is informed.

Frequently asked questions

Is it contagious to my other plants? It is a corn disease and will not move to unrelated crops. Within a field it spreads plant to plant on wind and rain splash, and infected residue carries it into the following season.

When should I treat? Fungicide is applied at early tassel stage, and only if disease pressure is high — the decision comes from scouting the lower leaves, not from the calendar. Protecting leaf tissue through grain fill is the point of the timing.

Can I eat corn after spraying? Observe the pre-harvest interval of 21 days for Azoxystrobin (Amistar). Because the spray goes on at early tassel and the crop is harvested considerably later, this interval is normally satisfied well before harvest.