Each year, the University of Georgia establishes sentinel plots across the state that include two corn hybrids, one which is susceptible to both races of Puccinia polysora (Southern rust pathogen) and one which is only susceptible to the new race of P. polysora. Leaf samples are collected weekly from each of these plots and are analyzed for rust diseases and also for leaf blights at the diagnostic clinic in Tifton. The results are distributed to county agents and also posted on the Internet at www.sbrusa.net.

“We establish plots, mostly in commercial fields, with rust-resistant lines planted side by side with non rust-resistant lines. We monitor those plots every week watching for Southern corn rust. About the 190th day of the year was the average first date we found the disease. That means for most counties, and for most of our sentinel plots, we started picking up both the resistant race the susceptible race on about July 9.

“At the end of the season, for the most part, it didn’t matter which variety you had out there, rust was on all of them. This means that the second race, which was new to us in 2008, is here to stay. In a few counties in the northeast part of the state, we didn’t have as much rust. But in the Coastal Plain, we had a lot of rust throughout the season. The resistant varieties are reducing the amount of rust, but not the incidence of rust — there’s still a lot of the disease out there,” says Kemerait.

Four years ago, fungicide options included Quadris, Stratego, Headline and Tilt, he says, but the choices have now increased. “We now have Quilt Xcel and Headline AMP. What’s nice about these products is that they are a combination of a protective fungicide with curative activity.”

Southern rust is a very aggressive disease of corn, says Kemerait.

Significant yield increases were not detected in every corn fungicide trial conducted by the University of Georgia in 2010. However, in at least one trial at the Stripling Irrigation Park in Mitchell County, a single well-timed application of Headline or Headline AMP increased yield by nearly 25 bushels per acre, he says. Other labeled fungicides were not tested at this location, but would most certainly have protected yield as well, says Kemerait. Fungicides also protected yield in other trials in the state.

“The fungicide either went out at first tassel or we came in at first silking, approximately two weeks later. At the end of the season, it looked bad no matter what was sprayed. But as far as yields, by spraying a single, well-timed fungicide at first tassel, we made 22 bushels per acre. The first tassel Headline and the first tassel Headline AMP were exactly the same with a 22-bushel yield increase.

“If you waited until silking, which may have been my recommendation watching the rust come in, you still made more corn, but you lost bushels by not making that first at-tassel application. And you didn’t lose as much with Headline AMP.

“By spraying a protective fungicide and delaying that application, you did not have the curative activity to protect that yield. By spraying one with curative activity, you had some leeway if you were a little bit late, and you protected the stalk.”