Northern leaf blight on corn — symptoms and treatment

Severity: high

What is Northern leaf blight

Northern leaf blight is caused by Exserohilum turcicum. It causes long, cigar-shaped gray-green lesions on leaves. Big lesions destroy a lot of leaf at once, and when the disease reaches the leaves around the ear while the grain is filling, the plant loses the factory it needs to finish the crop. That is why it ranks as a high-severity corn disease rather than a cosmetic one.

Symptoms

The lesions are large and unmistakable: long, cigar-shaped and gray-green, measuring 1-6 inches. They are elliptical with tapered ends, and — unlike gray leaf spot — they are not confined between the veins, so they sprawl across them and can span a good stretch of the leaf. Infection starts on the lower leaves and progresses upward, one storey at a time, so the vertical position of the disease tells you how far along it is. When lesions merge, whole leaves are killed.

• Early: one or two long gray-green lesions low on the plant
• Early: cigar shape, 1-6 inches, tapered at both ends
• Advanced: lesions merging, lower leaves dying
• Advanced: disease progressing up the plant toward the ear leaves

Gray leaf spot is the main confusion. Its lesions are rectangular with straight parallel sides because the veins box them in; Northern leaf blight lesions are cigar-shaped and cross the veins freely.

Causes and conditions

Exserohilum turcicum survives on infected corn residue. Spores form on that residue and are carried by wind and rain splash onto the lowest leaves — which is exactly why symptoms start at the bottom of the plant and climb. Infection requires extended leaf wetness, so heavy dew, frequent rain and humid nights drive it, and it is favoured by moderate rather than hot temperatures. Each new lesion produces spores that infect the leaf above, so a humid stretch can move the disease up the canopy quickly. Continuous corn with residue left on the surface is the highest-risk setting, since last year's inoculum is lying directly beneath this year's plants.

Treatment

Crop rotation — cultural

Timing: planting season. Rotate with non-host crops for at least 1-2 years. Use resistant hybrids.

Pyraclostrobin + Metconazole — chemical

Timing: tasseling stage. Apply fungicide at tasseling if lower leaf lesions are present and weather is humid. Pre-harvest interval: 21 days.

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

Prevention

• Rotate to non-host crops so infected residue breaks down before corn returns to the field.
• Use resistant hybrids, especially where the disease has appeared before.
• Manage residue rather than leaving a full mat of last year's corn on the surface.
• Avoid over-dense stands that keep leaves wet longer in the canopy.
• Scout the lower leaves from mid-season — their condition at tasseling drives the spray decision.

Frequently asked questions

Is it contagious to my other plants? It affects corn and will not spread to unrelated crops in the garden or field. It does spread readily to other corn, and infected residue carries it into the following season on the same ground.

When should I treat? At the tasseling stage, and only if lower leaf lesions are present and the weather is humid. Both conditions matter: lesions already on the lower leaves plus humidity means the disease has a route and a reason to climb into the ear leaves during grain fill.

Can I eat corn after spraying? Observe the pre-harvest interval of 21 days for Pyraclostrobin + Metconazole. With the application made at tasseling, harvest normally falls well outside that window.