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Diseases threaten Georgia corn

It's a rare sighting — a plant pathologist at a Georgia corn meeting. But it may become more common as diseases continue to cause problems for the state's corn growers.

“There's a perception out there, and it may or may not be true, that it's just not economical to control diseases of corn with fungicides,” says Bob Kemerait, University of Georgia Extension plant pathologist. “But growers are concerned about diseases in corn production, and some diseases have become more important in recent years.”

Corn fields in Georgia rarely are decimated by disease,” he adds.

“But that doesn't mean there aren't losses. Any time there are diseases in a field, there are losses. Severe losses might be restricted to specific fields, varieties or poor soil types.”

Disease losses can be minimized significantly by efficient management decisions, notes Kemerait. “Anything you can do to grow healthier corn will mean fewer disease problems. Using fungicides generally isn't considered an economic practice in Georgia corn, but we'll continue to look at this option in the coming years,” he says.

Even though diseases are considered less of a threat in corn than in peanuts, Georgia corn growers still are losing more than $10 million annually to diseases, says Kemerait.

The primary leaf or foliar diseases of Georgia corn are Southern leaf blight and rust, accounting for more than $3 million in losses, he says. Growers need to find economical ways of managing these diseases, says Kemerait.

Plant pathologists, he says, talk about a “disease triangle” composed of three parts - the host, which would be the type of corn variety; the pathogen, which would be the fungi or nematode causing the disease; and the environment.

“Environmental conditions are extremely important in the development of diseases. If you have the correct pathogen and the wrong host, you'll still have the disease if environmental conditions are favorable. If environmental conditions aren't favorable, you won't have the disease,” says Kemerait.

The most common causes of seedling diseases on corn in Georgia are rhizoctonia and pythium, he says. “One of the primary reasons for seedling disease is when we plant into conditions that don't favor rapid germination and growth of the seed. Anything that slows down germination and slows down vigorous plant growth will cause problems. Warmer soils are more favorable for promoting rapid germination and less seedling diseases.”

The first thing growers can do to prevent seedling disease is to plant quality seed, he advises. Most corn seed planted in Georgia already is treated with at least one fungicide, he adds.

“Rotation with non-grass crops is another step we can take to prevent seedling diseases. The rhizoctonia species that attack corn aren't the same ones that give us problems in cotton and peanuts. Rotating corn with a non-grass crop will give us fewer problems with seedling diseases.”

Many of the fungi and pathogens that cause diseases in corn survive between seasons, says Kemerait, so it's imperative that growers practice good field sanitation after a crop is harvested, including removing old crop debris.

Stalk rot, he says, is caused by several pathogens, with bacterial stalk rot being more common on Georgia corn. “It's very environmentally sensitive, and we see it more often in wet weather. It's common after tasseling, particularly when you've received excessive rainfall. Rainfall predisposes the stalk to rapid colonization by the pathogens that cause stalk rot.”

Stalk rot is managed primarily by IPM rather than by fungicides, says Kemerait, with crop rotation and proper fertility offering the best defense against the disease.

“You're less likely to have lodging if you have a good fertility program. Good fertility insures that plants will be strong and less susceptible to stalk rot. Anything that might cause a wound in the stalk, including insects and other pathogens, can lead to stalk rot. Also, nematodes can weaken the plant, making it more susceptible to stalk rot.

“Minimizing moisture stress also will help prevent stalk rot. In addition, timely harvest is beneficial. The more quickly you can harvest, and the less time corn spends in the field, the less likely you'll have lodging problems. There's evidence from other states that Bt corn may offer limited benefits because it may reduce the amount of insect feeding damage.”

Moisture is a necessary requirement, says Kemerait, for the development of foliar diseases in corn. Moisture is important for spore germination, he says.

“The most important factor — in addition to moisture — both for bacterial and fungal infection is warm temperatures. The warmer the temperature, the more likely you'll have good growing conditions for bacteria and fungi.”

Common rust, he says, generally is found on plants relatively early in the season and grows best in cooler temperatures. Southern rust is favored by the hot, humid temperatures that occur later in the season. There's a perception among growers, he says, that common rust now is more widespread in Georgia.

“Common rust and Southern rust both have been related to substantial disease losses, especially in the southwest region of the state. Growers are losing yields from these diseases, and that's one reason we're taking a closer look at using fungicides.”

Corn growers have seen more Southern leaf blight in recent years, says Kemerait. Southern leaf blight reduced yields this past year in the southwestern portion of the state.

“We saw a lot more Southern leaf blight in 2001 than in 2000, and this was repeated last year. There wasn't much difference in the temperature ranges in 2000 and 2001, but rainfall amounts were different. With a wetter and cooler spring predicted for this year, we can expect to see more seedling and foliar diseases early in the season.”

Southern leaf blight, he says, probably is the most important disease of corn in Georgia, with the exception of mycotoxins. “The best way to manage this disease in Georgia is with variety selection. There isn't complete variety resistance, but there is some. Crop rotation and field sanitation are very important in controlling this disease.”

Smut can be found every year in Georgia corn fields, says Kemerait. This disease prefers warmer temperatures, and when weather conditions are favorable, it can cause significant field losses.

Good insect management, crop rotation and variety resistance all can help to prevent smut, he says.

Nematodes also are a problem in Georgia corn, he says. “The same nematodes that build up in cotton and peanuts also may affect your corn. We're most concerned with root-knot nematodes. The Southern root-knot nematode goes to cotton and peanuts. Corn can be a ‘bridge’ for root-knot, Southern root-knot and peanut root-knot nematodes.

“We can use nematicides. But if you think you have a problem, take a soil sample and work with your county Extension agent or consultant to decide if it would be economical to treat for nematodes.”


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