Common rust on corn — symptoms and treatment

Severity: medium

What is Common rust

Common rust of corn is caused by Puccinia sorghi. It produces characteristic reddish-brown pustules on leaves. It is a familiar sight in corn and usually more alarming to look at than it is damaging, but a heavy early infection costs the plant green leaf area at the stage when it can least afford it. Sweet corn is generally hit harder than field corn.

Symptoms

Rust does not make spots — it makes pustules, and the difference matters. These are raised, powdery, reddish-brown ovals that break through the leaf surface and release spores; rub one and rusty dust comes off on your finger, which no leaf spot disease does. The pustules appear on both leaf surfaces, upper and lower, another useful identifying feature. As they mature the pustules darken to black, the pathogen switching to its overwintering spore stage.

• Early: small reddish-brown oval pustules scattered on the leaves
• Early: pustules on both leaf surfaces; rusty powder rubs off
• Advanced: heavy pustule coverage, leaves losing green tissue
• Advanced: pustules darken to black as they mature

The two-sided distribution and the powder that rubs off distinguish it from gray leaf spot and Northern leaf blight, which produce flat, dry lesions that stay put.

Causes and conditions

Common rust behaves differently from corn's residue-borne diseases. Puccinia sorghi does not overwinter reliably in cold climates on residue; instead its spores blow in from warmer regions and arrive on the wind, so the arrival date varies from year to year and is largely out of a grower's hands. Once spores land, infection needs leaf wetness — dew, rain, humid nights — and is favoured by moderate, not hot, temperatures. Cool, wet, humid spells therefore drive it, and it can move quickly through a field because each new pustule produces a fresh crop of wind-blown spores. The important variable for damage is timing: rust that arrives early, before tasseling, has time to build.

Treatment

Resistant hybrids — cultural

Timing: at planting. Plant rust-resistant corn hybrids. Early planting can reduce disease pressure.

Propiconazole (Tilt) — chemical

Timing: before tasseling. Apply foliar fungicide if rust appears before tasseling and conditions favor spread. Pre-harvest interval: 30 days.

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

Prevention

• Plant rust-resistant hybrids — genetic resistance is the cheapest and most reliable control for a disease that blows in.
• Plant early where possible, so the crop is further along before spores arrive.
• Scout regularly through the pre-tassel period, since the arrival timing decides whether treatment pays.
• Avoid dense stands that hold dew and humidity in the canopy.
• Grow sweet corn with airflow in mind — it is the more susceptible type.

Frequently asked questions

Is it contagious to my other plants? Puccinia sorghi is a corn pathogen and will not infect your beans, tomatoes or squash — the rusts you see on those are different fungi entirely. It spreads to other corn on the wind.

When should I treat? Only when rust appears before tasseling and conditions favor spread. Pustules showing up late, on a crop already well past tasseling, generally do not justify a spray — the plant has already done most of its work.

Can I eat corn after spraying? Observe the pre-harvest interval of 30 days for Propiconazole (Tilt). This one is worth planning around on sweet corn, where harvest can follow a pre-tassel application more closely than in field corn — check the interval will be satisfied before you spray.