“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).