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Corn+Soybean Digest

Northern Leaf Blight Threatens Georgia Corn

Georgia Extension Plant Pathologist Bob Kemerait warns corn growers to get ready to fight damaging diseases in their 2010 crops. He says southern rust has been a concern for a number of years, especially in hybrids with little or no resistance. Northern corn leaf blight is the newest corn disease threat, according to Kemerait. And he remains concerned that nematodes will continue to deplete corn yields.

“Corn disease wasn’t much of a concern prior to 2005,” he notes. “Since then, we’ve seen diseases become more threatening to our corn, and we’ve seen an increase in the number of fungicides available to fight these diseases.” Some of these available fungicides include Tilt, Folicur, Stratego, Quadris, Quadris Xcel, Quilt and Headline.

Kemerait and his colleagues maintain a series of sentinel plots throughout Georgia. These corn plants provide an early indication of the types of corn diseases and severity in any given season.

“Southern rust can devastate non-resistant hybrids,” says Kemerait. “Southern rust came in late during the 2009 growing season. Southern rust has been the No. 1 disease on Georgia corn until last year. Southern rust tends to be bad about every five years. The spores don’t over-winter here, but they are carried in the winds from Mexico, the Caribbean and Central America.”

Though many corn hybrids have good resistance to southern rust, a new race of the disease showed up in 2008 that successfully attacked the resistant hybrids. Overall, 2009 was a mild year for southern rust. The new race of southern rust reappeared last year, though damage from it was minimal.

For 2010, Kemerait advises corn growers to scout fields carefully, monitor reports from the sentinel plots and be ready to spray a fungicide if the southern rust threat intensifies.

Last year, northern corn leaf blight overtook southern rust as the primary corn disease threat in Georgia. Kemerait saw one field in Macon County, Ga., produce only 70 bu./acre as a result of this disease, even though the field was fertilized, irrigated and managed to produce 200-bu. yields.

“I can almost guarantee that northern corn leaf blight will be a problem in 2010,” says Kemerait. “The spores are out there now over-wintering.”

He wonders if new hybrids are being screened sufficiently for tolerance to northern corn leaf blight. “You are more likely to have this problem where you plant corn behind corn,” he says. “Corn prices and the yield potential of your crop will determine if a fungicide will pay off in controlling northern corn leaf blight. We know that with southern rust, one application of fungicide can get you more than 10 bu. of grain/acre in higher yields. But I can’t give you a figure on how many extra bushels you’ll produce if you spray for northern corn leaf blight.”

Getting a positive response from fungicides on this disease will depend in large part on selecting the best fungicide, according to Kemerait. “I’d suggest using a strobilurin fungicide or a strobilurin-triazole mix,” he adds. He suggests timing the application for the period when corn plants approach the tasseling stage. “Also, before you spray, make sure it is northern corn leaf blight, not southern corn leaf blight.”

Kemerait also addressed nematode control in corn. He says root-knot and stubby-root nematodes are the main concerns for corn growers. Nematicide testing in recent years confirms that yields are likely to increase when infested fields are treated. Fumigation with Telone II showed benefits in his 2009 tests. Fumigation with Telone II prior to planting helped corn growth and led to good yield increases where nematode populations caused significant damage to the crop.

A trial in Burke County, GA, resulted in improved control using either Telone II or Counter. One of these tests with Telone showed improved nitrogen utilization as a result of treatment. “We’re not saying you should cut nitrogen use when you apply Telone II, but we do see better roots as a result of treatment and the better roots make better use of nitrogen,” he explains.

New seed treatments for nematodes may also help corn growers. Kemerait reported on his Avicta Complete seed treatment tests in which corn results mirrored those obtained in cotton. “The seed treatment seems to be doing something good,” he says. “We saw better plants where Avicta Complete was used than where Cruiser insecticide alone was applied. These plants from the Avicta treatment appeared similar to plants receiving a Counter treatment. Counter 15G is the standard treatment for nematodes in corn. So we can take some confidence in suggesting seed treatments for fields with low to moderate nematode infestations.”

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