At a time when the sky would appear to be the limit for Southeastern corn yields, managing diseases becomes even more important, meaning a yield difference of as many as 20 to 30 bushels per acre in some fields.

One of the biggest disease threats to corn growers — Southern rust — was practically non-existent last year due to hot, dry weather conditions, says Bob Kemerait, University of Georgia Extension plant pathologist.

“It was the first year since 2005 that I haven’t made wide-scale recommendations for spraying fungicides for high-yield producers,” says Kemerait. “It was hot and dry, and there wasn’t a way for Southern corn rust to get here — it needs a tropical storm to get to our fields.”

Southern rust, he says, is the most important disease faced by Georgia corn producers. The presence of the disease in the state is monitored in sentinel plots, he adds.

“We looked for it every week of the season but didn’t find it,” says Kemerait. “The most important aspect of the disease is that once it has sucked the juice from the leaf, it’ll start to suck it out of the stalk. When you invest in a fungicide to prevent Southern rust, you’re protecting against lodging late in the season.”

Severe Southern rust outbreaks, which typically occur every three to four years, have been linked with up to a 50-percent reduction in anticipated corn yields, says Austin Hagan, Auburn University Extension plant pathologist.

“Crop growth stage greatly impacts disease effect on yield and the need for protective fungicides,” says Hagan. “If rust shows up in corn in the tassel stage or silking, sizable yield losses are likely given favorable weather for disease development and lack of protective fungicide treatments. Double-crop corn is a sitting duck for Southern rust.”

The best way to manage Southern rust, says Kemerait is to be out in the field looking for it, or have someone else look for it. “The problem is we have two rust diseases — Southern rust, which matters, and common rust, which doesn’t matter. The best way to tell them apart in a field, if you’re scouting, or if you have someone looking for you, is that if you have orange pustules, and they are on the top and the bottom of the leaf, don’t worry about them. But if they’re only on the top of the leaf, it might be Southern rust.”

Southern rust pustules, which are distinctly orange in color, are circular to oval in shape and smaller compared to common rust, says Hagan.

Other symptoms to look for

“Southern rust pustules appear in tight clusters on a portion of the leaf, while those of common rust tend to be scattered across the leaf. A light green to yellow halo or border may surround newly formed pustules.

“While pustules are so numerous they may cover the upper leaf surface, heavy pustule formation may be seen on the leaf sheaths, stalks, ear husks and, to a lesser extent, on the leaf underside along the mid-vein,” he says.

Spores of Southern rust can move on wind currents from Cuba, Mexico, Central America and south Florida.

Resistant varieties, says Hagan, are not a particularly practical control for Southern rust. Also, a new race of the Southern rust fungus can partially defeat this resistance.

When rust pressure has been high, yield gains of 20 to 30 bushels per acre have been obtained with timely fungicide treatments, according to Hagan. However, yield gains with fungicides in the absence of significant rust pressure have been erratic.

“Blindly applying fungicides to corn without regard to disease pressure, crop growth stage, and yield potential is a waste of money,” says Hagan.

“To best target fields in need of protection, farmers need to scout or have a consultant check their fields, particularly those under irrigation, for early symptoms of Southern rust as well as other diseases like Northern corn leaf blight.”

The language on several fungicide labels specifies that the first application should be made when “symptoms first appear,” he says.

“For preventative rust control, make the first fungicide application at tasseling to silking and follow with a second as needed application about 14 days later if favorable weather patterns for disease development continue. Once corn reaches soft dough, further fungicide treatments are not needed because rust no longer will reduce yield.”

Due to lower yield potential, says Hagan, it’s harder to justify the additional cost of a fungicide on dryland compared with irrigated corn.

Another disease of concern to corn growers is Northern corn leaf blight, says Kemerait.

“In 2008, I saw it for the first time, and I now have a huge amount of respect for it. Northern corn leaf blight will be most severe in corn behind corn, when you have adequate moisture, and when you plant varieties that are more susceptible to the disease,” he says.

For detecting Northern corn leaf blight, Kemerait advises growers to look at the leaf located three leaves below the ear leaf. “If you have Northern corn leaf blight developing three leaves below the ear leaf, that’s the time to take notice,” he says.

The disease can be as important as Southern corn rust, he continues, and it has been associated with significant yield loss.

“How do we anticipate it? My target for spraying is first tassel. It’s a moving target, but that’s the ideal time. The problem with Northern corn leaf blight is that that may not be early enough.”

While growers have an excellent arsenal of fungicides at their disposal, Kemerait says yield potential should be a guide when using them.

“If you’re a dryland farmer with a yield of 70 bushels, you don’t need to think about fungicides unless it’s an exceptional year. If you invest in corn seed, irrigation, fertilization, weed control, and other aspects, you don’t want to let disease take anything away from you.”

(Diseases, nematodes and aflatoxin all have become increasingly important factors in corn production, especially as rising prices have made it more feasible to treat for such problems. These are all critical production aspects faced by Southeast growers. To see the latest control recommendations, visit Corn diseases becoming bigger production factor).

phollis@farmpress.com