Diseases more of an issue in Georgia corn

There was a time when Georgia corn growers didn’t give much thought to diseases or nematodes. But now, with the state’s acreage hovering between 350,000 and 400,000 acres and prices soaring, they could become major issues.

“We’re building up our acreage in corn and there’s a lot of interest,” says Bob Kemerait, University of Georgia Extension plant pathologist. “The increased acreage will affect our rotations, and that could bring good things and bad things.”

There will be nematode issues with rotation, but there also will be diseases such as Southern corn leaf blight, Southern rust and others, he adds.

Along with controlling diseases and plant parasitic nematodes on corn, a lot of emphasis now is being placed on using fungicides to protect yield and maintain plant health, says Kemerait.

“For a long time, we haven’t focused on nematodes in field corn because we haven’t been looking for them. So what’s out there for managing nematodes? We’ve been using Counter to treat nematodes affecting corn in the past. Recently, we have seen some very strong responses to using Telone II. There’s also a new nematicide on the horizon — Avicta Complete Corn,” he says.

There are some diseases, says Kemerait, that will affect corn every year you plant, such as Southern corn leaf blight. “If we were in the early 1970s, it would be a major issue, but in 2008, it’s not such a major issue. But it’s out there. We also have Southern rust and common rust, and it’s important we know how to distinguish between those two diseases,” he says.

Some diseases that affect field corn every year have the potential, he adds, to reduce grain fill, thereby reducing yield. “But they also have the potential to increase damage to your stalks, affecting stalk lodging and ultimately yield,” he says.

As far as managing diseases of field corn, there are resistant varieties and several fungicide options, says Kemerait.

The primary disease concern in Georgia is Southern rust, he says. “Not that the other diseases don’t matter, but for corn growers in Georgia — from what we’ve seen — Southern rust is the No. 1 disease problem that can steal yield from you. Unless you have a resistant variety, you’ll get yield losses if you have Southern rust in your field at a critical time.”

Spore production from Southern rust is extreme, says Kemerait, and it’s important to tell the difference between Southern rust and common rust.

“If you have common rust, I don’t believe you need to worry about it. But if you have Southern rust, you should be worried about it. The easiest way to tell them apart — unless you have a microscope — is that Southern rust pustules are on the top of the leaf, and common rust pustules are on the top and the bottom of the leaf. In Southern rust, the pustules are the color of Tennessee orange. But in common rust, they look like Auburn orange. In the field, they can look the same, so it’s important to get samples to your county agent or consultant for identification,” he says.

Plant health is becoming more of an emphasis now in field corn, especially from fungicide companies, says Kemerait, and fungicides like Headline and Quadris do affect physiological responses in a corn crop.

“The effects are more obvious in soybeans, where the plant will remain green for a longer length of time than if you didn’t use the strobilurin fungicides. The question is, does that physiological response, which makes the plant appear healthier, produce greater yields? Because you’re a corn grower, you have the opportunity to buy products for plant health in the absence of disease. I’m not against that concept, but I don’t want you to buy anything unless you’ll make more money from using it.”

There are several fungicides labeled for field corn, including Tilt, Stratego, Quadris, Quilt and Headline, says Kemerait. “We have not done that much work with Tilt, and it’s probably the weaker of the fungicides available for field corn. But the others are all good, strong fungicides. The advantage Stratego might have is that it’s a little less expensive than the others, but the others may be a little stronger than Stratego.”

Georgia researchers began conducting on-farm fungicide trials on field corn in 2003. The trials have all been replicated and treatments were made with aerial application. The rates used were the ones recommended by the chemical companies. In 2003, 2004 and 2005, all applications were initiated when disease was found in the field.

“When the companies talk about plant health, they say you need to start that application earlier, at first tasseling. Applications we made were at the first sign of disease, typically during late silking stage. For plant health, they talk about getting it out earlier, and that’s what we did in 2007.”

In a test conducted on a field in Terrell County in 2003, researchers looked at the effect of fungicide applications on resistant and non-resistant corn hybrids. “Dekalb 687 typically does not have the yield potential of Pioneer 31G98 or 32W86, but it does have better resistance to foliar diseases. With one or two fungicide applications to Dekalb 687, we didn’t see any yield benefits in the presence of southern rust. But when you turn to 32G98 or 32W86, which have less resistance but more yield potential, you can see a response from spraying fungicides — with a single or a dual application. When we averaged that yield, with Southern rust in the field, we saw an increase of 10 bushels per acres for each application of a fungicide. That was a success.

“The greatest economic return for 687 was when we did not apply any fungicide to it. But looking at 32W86, a single application of a fungicide bumped its profit above what we got with the 687 and two applications took it to the highest value we had. With Southern rust, we see a yield and value advantage when we apply fungicides to susceptible varieties.”

Trials were conducted in Seminole and Terrell counties in 2004 where Southern rust wasn’t present in the field. “We get Southern rust in a crop maybe once every three years, but Southern corn leaf blight is always out there. When we treated for Southern corn leaf blight, we didn’t see the major yield advantage. By in large, with Southern corn leaf blight, we don’t see much of a yield advantage, if any, from spraying fungicides.”

But as far as plant health is concerned, the applications in these trials were made at silking or at the first appearance of disease rather than at tasseling, which is recommended by agrichemical companies for plant health, says Kemerait.

“In trials conducted in Blakely, Jenkins and Burke counties in 2007, at the end of the season — looking at untreated checks — I saw no reason, based upon disease, to spray. I would not have advised the growers to use fungicides in those cases to protect yield from disease. In the absence of Southern rust, I can’t guarantee you a yield benefit from spraying. With plant health, I can’t say for certain. Some trials have shown nice results. I will say this — if you spray a fungicide for plant health on field corn in Georgia, in the absence of disease, don’t count on a yield increase. I can’t say you won’t get one, but it’s not automatic. When you make your decision, look at the price of corn and the potential for disease to come into your field. I’m not saying the plant health benefit is not real. You just can’t say that a fungicide will automatically increase yield.”

If you’re irrigating and have good yield potential, using a fungicide is an insurance policy, says Kemerait. “We have high corn prices, so it’s a good time to protect your yield potential. If Southern rust isn’t present, weigh the considerations of Southern rust disease pressure versus the plant health and how much you want to invest in that.

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