Southern Rust Rising

Tropical Corn Disease Heading North Earlier

Emily Unglesbee
By  Emily Unglesbee , DTN Staff Reporter
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Southern corn rust pustules are mostly found on the upper side of corn leaves, as opposed to common rust, which shows up on both the upper and lower sides. (Photo courtesy Bob Kemerait, University of Georgia Extension)

ROCKVILLE, Md. (DTN) -- The rusty orange dust that coated Scates Farm's combines last fall was as damaging as it was messy and unexpected.

The dust came from the Southern corn rust disease that swept through southern Illinois in August and damaged the Scates' late-season corn hybrids -- nearly one-third of their total corn acreage, Jeff Scates said. Test weights tumbled and he estimated the disease stole up to 10% of their yields.

Southern corn rust took farmers in Illinois, Missouri, Indiana, and Nebraska by surprise last year and may do so again in the years to come, plant pathologists told DTN. As its name suggests, the disease blows up from the tropics and doesn't always make it north of the Mason-Dixon Line. But for the last two years, it has reached Southern states a month earlier than normal. The result is more damage to Southern cornfields and a greater chance of infestation for Northern neighbors.

RUST ON STEROIDS

Southern corn rust is related to common rust, but it behaves far more aggressively in the field, said Southern Illinois University crop scientist Jason Bond. "It moves twice as fast as the fungal diseases we typically see in the Midwest," he told DTN.

Once a spore lands on a corn leaf, it takes only seven days for a lesion to develop, full of spore-bearing orange pustules. That means Southern rust infections can race across a cornfield before a farmer even knows it's there.

"You'd come over a hill, and look down into a corn field that was colored almost completely orange from all the spores," Bond said of last August in southern Illinois. "The amount of spores it would take for the entire field to have that orange color just boggles my mind."

These rust pustules are found on corn leaves, husks, and stalks and can lower grainfill, but the greatest damage may result from lodging, said University of Georgia Extension plant pathologist Bob Kemerait.

"Once the leaves are starved, the ear will pull nutrients from the stalk to feed the ears," he explained. "You end up with poor-quality stalks and then lodging becomes a problem. When the disease is severe, we've seen up to 80 to 90 bushels lost."

WATCH AND WAIT

By the time southern Illinois farmers knew what had hit them last August, it was too late to treat fields, Bond said. Warm, humid weather with plenty of rainfall gave the disease a boost, and fields seemed to become infested "almost overnight," Bond recalled.

"In hindsight, all the warning signs were there, though," he said.

Plant pathologists in Southern states like Georgia had begun posting alerts of the disease's arrival as early as June. "The last two years it has shown up extremely early -- June 6 and June 5," Kemerait said. "That's three to four weeks earlier than normal and adds tremendous pressure on the corn crop."

Midwestern growers would do well to keep tabs on disease reports from their Southern neighbors this year, Bond said. Late-planted cornfields, and long-season corn hybrids will be most at risk. "Every day you delay corn planting, you're inching closer to rust arrival and the more severe the epidemic will be," Kemerait said.

Fields that are prone to corn diseases -- low-lying river bottoms and shaded fields, for instance -- merit an especially close watch, Bond said.

Corn hybrids with good resistance to Southern rust are not readily available, but you can knock some stress off the plant by selecting hybrids that will resist other corn diseases like gray leaf spot and Northern corn leaf blight, Bond added.

SCOUT OFTEN, SPRAY EARLY

A well-timed fungicide application that includes some combination of strobiluron, triazole and SDHI fungicides will more than pay for itself in bad Southern rust outbreaks, Kemerait said. But because the disease moves so fast, getting it on in time can be a challenge.

Discard the threshold recommendations that come with other corn diseases, Bond advised. "If you find a single pustule of Southern rust on the upper side of one leaf, and it's early enough in the season, between VT (tasseling) and R4, and environmental conditions are right, you need to spray," he said.

The worst thing a grower can do is mistake Southern corn rust for its slower-moving, less lethal cousin, common rust, Bond said. Common rust will form pustules on both the upper and lower side of corn leaves, whereas Southern rust typically only infects the upper side. "I describe the color of common rust pustules as more auburn or burnt orange," Kemerait said. "Southern rust pustules are more of a yellow-orange."

If you're in doubt, send samples to a lab, where the shape of the spores will make for easy expert identification, he said.

EYES TO THE SOUTH

Above all else, be aware that a shifting climate and more volatile storm systems means Southern corn rust could be a more frequent visitor in a greater range of the U.S., both Bond and Kemerait warned.

The disease doesn't typically overwinter in the U.S. -- new spores have to blow in from Mexico and the Caribbean, but that could change.

In Georgia this year, unseasonably warm temperatures kept volunteer corn hosts growing into mid-January, Kemerait said. Cold temperatures finally killed them and any rust spores they might have housed late in the month, he said.

"But what about Florida?" he said "Did it survive there? The quicker it gets to Georgia and Alabama next year, the sooner we have an epidemic, and the sooner we send the spores north."

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

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Emily Unglesbee