Fight Foliar Diseases

LAWRENCE, Kan. (DTN) -- Regular rains this spring and summer have set up many Corn Belt farmers for a bumper crop this year, but corn plants aren't the only beneficiaries of the weather. Moisture-loving foliar diseases like northern corn leaf blight and gray leaf spot are also thriving in many Midwestern states.

"I think this is going to be an interesting year for corn foliar diseases," University of Missouri plant pathologist Laura Sweets told farmers at a pest management field day in Columbia, Mo., last week.

Northern corn leaf blight is more common in the northern Corn Belt states like Iowa, Minnesota, Michigan, and Wisconsin, but growers as far south as Missouri are seeing both this fungus and gray leaf spot this year, Sweets said. Both foliar diseases favor wet corn canopies, but future temperature trends will determine which outbreaks get worse, she said.

"If we stay on the cool side, we may see more northern corn leaf blight, but if we start to warm up, the gray leaf spot and diseases like rust could take off," Sweets said.

NORTHERN CORN LEAF BLIGHT

Iowa State plant pathologist Alison Robertson recently reported numerous outbreaks of northern corn leaf blight in Iowa, particularly in western and central parts of the state.

"Considering the warm, humid conditions we have been experiencing, it is no wonder NCLB is developing in fields," Robertson wrote in the university's Integrated Crop Management newsletter. "What is a little unusual is to hear reports of the disease occurring in Iowa prior to tasseling."

When scouting cornfields for this disease, look for "long, canoe-shaped or cigar-shaped lesions that can be anywhere from 1 and 2 inches to 6 and 7 inches in length," Sweets told growers.

Robertson noted that the tan lesions can easily be confused with other diseases, like Goss's Wilt, and recommended this Iowa State Integrated Pest Management article to help growers distinguish between the two: http://goo.gl/….

For growers with significant northern leaf blight in their fields, Robertson recommended using a fungicide containing triazole.

While no specific thresholds have been established for northern corn leaf blight, most pathologists agree that protecting the ear leaf and leaves above it is most critical for preventing yield loss. Keep in mind that the timing of mid-season fungicide applications is also very important, Robertson said.

"Farmers that have NCLB in their field should wait to apply fungicides until tasseling since applications between V12 (12th leaf) and VT (tasseling) may cause arrested ear development, but if the cool, wet weather continues, fungicide applications should not be applied too late," she wrote.

As plants continue to develop, farmers should also consider the cost-to-benefit ratio of spraying, Sweets added. "Once you get past about R2 (blister), you're starting to see less of an impact from some of these diseases and perhaps less of a benefit to foliar fungicide applications," she said.

GRAY LEAF SPOT

The window for effectively treating gray leaf spot in corn is narrow and closing, Kansas State plant pathologist Doug Jardine warned growers in a university Extension Agronomy Update last week.

"Most corn fields in Kansas are now, or soon will be at the critical juncture for making fungicide application decisions for gray leaf spot management," he wrote. "Years of fungicide application research clearly demonstrates that the single best time to apply a fungicide to corn for gray leaf spot control is from VT (tasseling) to R1 (silking)."

Look for lesions that are smaller and more rectangular than northern corn leaf blight, Sweets said. "The [gray leaf spot] fungus doesn't grow across the veins of the leaves, so you get these lesions that are long, rectangular and have the very straight edges," she said.

Because it survives in corn residue, growers who grow corn after corn have a higher risk for the disease this year, she added. However, anyone with good canopy moisture could be at risk for gray leaf spot.

"It is favored by warm temperatures and moisture in the canopy -- either from frequent rains, from heavy dews, or from overhead irrigation, and then temperatures in the 75- to 90-degree range," Sweets told growers.

Spraying thresholds vary by hybrid type, Jardine noted. For susceptible varieties, fungicide applications are recommended if the disease can be spotted on the third leaf below the corn ear and higher on 50% of scouted plants. For hybrids with an intermediate disease resistance ranking for gray leaf spot, Jardine adds these criteria to the threshold: a history of foliar disease problems in the field, corn replanted after corn, 35% or more surface residue, warm, humid weather.

When scouting for these diseases and others, beware of lesion mimic, a genetic abnormality that resembles many corn diseases, Sweets said.

"We're starting to see a little bit of [lesion mimic] from the central Missouri area," she warned growers. "You'll see these smalls spots that kind of run together into yellow streaks. It looks a little bit like diseases, but it's actually a genetic trait in the corn hybrid that may be expressed under certain environmental conditions."

See pictures of lesion mimic alongside real corn diseases in this Iowa State article by Robertson: http://goo.gl/….

Be especially vigilant in scouting late-planted cornfields for foliar diseases, Sweets said. "Late-planted corn is going to be behind in its growth stages, and that inoculum is starting to build up, so that corn will be at greater risk," she said.

Overall, farmers should take a variety of factors into account when weighing fungicide decisions, she concluded. "As you're talking ... about applying foliar fungicides, look at how much disease is already occurring, what are your weather conditions, what was the date of planting and the stage of growth of the corn, and make your decisions from there," she said.

The Corn Disease Working Group produced a spraying guide that compares fungicide efficacy for a variety of foliar corn diseases, including northern corn leaf blight and gray leaf spot. You can find it at this Purdue website: http://goo.gl/….

Emily Unglesbee can be reached at emily.unglesbee@dtn.com

Follow Emily Unglesbee on Twitter @Emily_Unglesbee.

(PS/CZ)

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