Gray leaf spot vs. northern corn leaf blight vs. southern rust

Comparison table

Gray leaf spot

Look for the first lesions on the lowest green leaves two to three weeks before tassel, in the low, still parts of the field and along tree lines where dew lingers. Early lesions are small with a yellow halo that shows when you hold the leaf up to the light; within about two weeks they lengthen into the classic rectangle, 0.2 to 2 inches (5–50 mm) long, and the fungus produces a new crop of spores that moves the disease up to the middle and upper leaves. Mature lesions are pale brown or gray to tan and so sharply bounded by veins that the edges look ruled. Where lesions coalesce they kill whole leaves.

Gray leaf spot needs warmth and long wet periods: 70–85 °F (21–29 °C) with relative humidity above 90 percent for 11 to 13 hours or longer. Rainfall itself matters less than hours of leaf wetness. The fungus survives on residue at or above the soil surface, through at least two winters under no-till in Kansas, which is why severity is greatest in continuous no-till corn and why rotating to soybeans reduces but does not eliminate the inoculum. Kansas State reports losses of 20–30 percent in individual fields with susceptible hybrids under reduced tillage.

Management. Hybrid resistance is the first line of defence. Tillage and rotation cut local inoculum. For fungicide decisions on susceptible hybrids, Kansas State's rule of thumb is to spray when lesions have reached the third leaf below the ear leaf on 50 percent or more of plants, scouting from two to three weeks before tassel; on moderately susceptible hybrids the lesions need to reach the second leaf below the ear and the field should have a disease history, and on highly resistant hybrids a fungicide is generally not economical.

Northern corn leaf blight

Lesions are unmistakable once mature: long elliptical blotches 1 to 6 inches (2.5–15 cm) long, gray-green at first and then pale gray or tan, running across veins rather than between them. In humid weather the fungus sporulates on the lesion, mostly on the underside, giving it a dirty gray, almost sooty look that distinguishes it from a dead patch of leaf. On hybrids with Ht single-gene resistance the lesions are smaller, yellow and may be only linear streaks, which can confuse the picture. Lesions can also form on the outer husk, although the kernels are not infected.

The fungus favours moderate temperatures of 64–81 °F (18–27 °C) with more than six hours of rain or dew, so it tends to show up in cool, wet summers and on irrigated fields. It overwinters in corn residue and spreads by wind and splashing water. At least four races exist; Ht-gene resistance protects against some races and not others, while polygenic resistance covers all races but is less complete.

Management. Choose hybrids with polygenic resistance where the disease recurs, rotate, and reduce surface residue where that fits the system. Several foliar fungicides are labelled; Crop Protection Network (CPN) does not publish a numeric threshold, so apply the same logic as for gray leaf spot: disease present on the leaves below the ear at VT, a susceptible hybrid and a favourable forecast.

Southern rust

Early infections are small circular to oval spots with a light green to yellow halo. They erupt into light orange to cinnamon-red pustules that are smaller than common rust pustules, round rather than elongate, very numerous and almost entirely on the upper leaf surface; pustules also appear on sheaths, husks and stalks. Rub one with a fingernail and the spores come off orange. Late in the season the pustules turn chocolate brown to black and form a ring of dark teliospores around the original pustule, which is when they get confused with tar spot. Common rust, by comparison, has larger and more elongate pustules on both leaf surfaces and is favoured by cooler weather averaging around 70 °F (21 °C), while southern rust takes off at average temperatures around 80 °F (27 °C) with high humidity.

Puccinia polysora does not overwinter in Nebraska or elsewhere in the northern Corn Belt. Urediniospores arrive on wind from the Deep South, typically early to mid-July in Nebraska, and a new generation of pustules forms about every seven days, so a field can go from a few pustules to blighted leaves within a few weeks of favourable weather.

Management. Resistant hybrids are the most cost-effective control for field corn. The fungicide decision is a timing problem: watch the Corn ipmPIPE map for confirmed counties to your south and scout the upper canopy once it is within a state or two. Foliar fungicides are effective and numerous products are labelled; the earlier the disease arrives relative to grain fill on a susceptible hybrid, the more yield is at stake and the stronger the case for a VT–R2 application.

Choosing a fungicide for any of the three

Products with activity on all three diseases come from FRAC Group 3 (DMI triazoles, e.g., propiconazole, prothioconazole), Group 7 (SDHIs, e.g., fluxapyroxad, pydiflumetofen) and Group 11 (QoI strobilurins, e.g., azoxystrobin, pyraclostrobin), usually as premixes. The Corn Disease Working Group rebuilds the CPN efficacy table every year from multi-year, multi-location field trials and rates each active ingredient disease by disease, so check the current table for the disease you actually found rather than assuming a product rated for one covers the others equally. Rates, intervals and preharvest intervals are on the label registered in your state or province. In Ontario, only products with a Health Canada PMRA registration (PCP number) listing the disease may be used; a US registration does not carry over.