From all the information available, the earlier a corn planting goes in, the less likely it will have issues with southern rust.
In a test conducted in Alabama this past summer in May-planted corn with two applications of Priaxor, yields were 150 bushels per acre versus 109 bushels with no treatments. In Georgia, there was a 70-bushel yield decline between the non-treated and the best fungicide treatments, says Austin Hagan, Auburn University Extension entomologist.
“We’re beginning to generate some information on corn varieties, and the problem we have at this time is that we had one gene in the marketplace that conferred a fairly high level of rust resistance in corn varieties. It was found in only one line, and that line is no longer available. But we have ideal conditions for rust, and all varieties are sitting ducks whenever you have this much rust inoculum," he said.
Another test was conducted in Brewton, says Hagan, which is about 70 miles northeast of Fairhope, Ala.
“Rust pressure isn’t as great as in Fairhope, but some of the varieties had a fair amount of rust on the flagleaf. We did have one variety – a Pioneer tropical corn 3035HR, and it basically had no rust on it. It’s the only variety around here I know of that has the rPP9 gene in it, but there’s a little yield drag with that gene. The problem is that we now have a race of the rust fungus that is virulent on varieties with this gene, so it’s not as good as it once was. It’s now more ‘slow rusting’ rather than resistant.
Rust race confusion
The problem with corn varieties, says Hagan, is that researchers can’t really figure out which rust races they have anymore because corn lines originally used to identify races of southern rust have disappeared.
There are a lot of fungicides currently available, he says, and there are more being registered all the time for use on corn.
“There’s not a lot of information from field trials on the performance of these materials with respect to timing. I suggest that growers use a scouting program. Look for symptoms, usually starting around VT and come back with a second application if needed. When the corn is tasseling, go ahead and make the first application and follow with the second one about two weeks later. There are calendar programs that suggest earlier applications at V6 or V8 and coming back at VT or the blister stage to make the second application.”
Growth stage trials also were conducted this past year, says Hagan. At a test in Fairhope, with an application at V6, there was a little bit of reduction in rust, he adds.
“As it got later in the season, control got a little better with a single application at VT. And we saw even better rust control with two applications. There really wasn’t that much difference in yield response as far as timing or the number of applications. We almost got as much of a yield response treating it at V6 than with the two applications.
“There was about a 50-bushel yield decline in this trial caused by rust. Stratego, Headline and Quilt Xcel are about the best treatments available. When rating rust, look at color – the more yellow it is, the worse the rust is. In another trial in Fairhope, some V6 applications didn’t get a reduction in rust, but we did see one at VT, and the best treatment was two applications of a high rate of Priaxor, where there was virtually no rust at all. The later the sprays, the better the control, and the more consistent the yield gains.”
Fungicide control varies
There also are differences in fungicide effectiveness, notes Hagan. “Just because they’re registered doesn’t mean they’re equally effective. Generic products in the Brewton trial planted in May did not work very well. Yields were the same as non-treated controls.”
Looking at the summaries of all trials, single applications actually did better than expected, says Hagan. “The V6 applications slowed down the rust so that yields actually increased a bit. Fungicide choice did matter. It looks like generics aren’t performing very well. Hopefully after another year’s data, we’ll start to see some differences in the other products.”
Alabama growers face rust outbreak
While southern rust isn’t an annual problem in every region of Alabama, producers saw plenty of it this past, when it reached outbreak proportions.
“It’s not an every-year disease in all areas of the state,” says Austin Hagan, Auburn University Extension entomologist. “Even in Baldwin County, which is the southern rust center of the state, we see problems only about every three to five years. If you look at double-crop corn, and we do have people along the Coast who plant corn after corn or corn after wheat, it’s almost an annual occurrence, and it’ll get into the second corn crop pretty badly.”
In Baldwin County and along the Florida Panhandle, southern rust showed up when the corn started to tassel, which is typically the window where it’ll do most damage to corn, says Hagan. The later it shows up, the less likely it is to have an impact on yield.
“There was enough rust in the lower counties to treat, especially for those growers who made good yields. For regular, early corn, there was no rust in central Alabama and none in the northern part of the state. However, if corn was planted a little later, then it was affected by rust,” says Hagan.
There’s always a question as to how much impact the disease has on yield, he says. “The earlier it shows up, the more likely it will have an impact on yield,” he says. “Pretty much the entire corn plant is sensitive to the southern rust fungus. We do have common rust on corn, and the pustules are a different color. They show up earlier in the season, and the disease doesn’t move nearly as quickly as southern rust does.”
Southern rust doesn’t overwinter in Alabama, coming in either from the Caribbean or Mexico, says Hagan. “We need certain types of weather patterns, particularly a frontal zone with a low pressure system moving out of the Gulf moving out of Mexico and coming into Alabama to bring a spore cloud into this area.”
Another possibility, he adds, would be an early season tropical storm that would bring inoculum into this area.
“The fungus is active during the winter in extreme southern Florida, so there’s actually a different track for that disease to move up into Georgia and into the Carolinas.”