About a third of Arkansas' roughly 3-million acre soybean crop is still in danger from exposure to Asian soybean rust, according to Cliff Coker, Extension plant pathologist, University of Arkansas Southeast Research Center, Monticello, Ark., speaking at a University of Tennessee soybean field day in Milan, Tenn.
The million acres comprise the state's double-crop soybeans planted after wheat. Coker added that on late, dryland fields where droughty weather in August significantly reduced yields, the situation may not warrant a fungicide application. But healthy beans with a good yield potential need to be protected.
Soybean rust was found for the first time in Arkansas in Little River County on July 22 in the southwest corner of the state, during a time when the disease was moving up through Texas. Coker noted that in that part of the state, rainfall was running around 15 inches above the rest of the state, and conditions were ideal for development of the disease.
“Fortunately, there are not a lot of soybeans grown there at the corner of Arkansas, Texas and Oklahoma.” Rust was also found in a sentinel plot in Hempstead County, about 30 miles northeast of Texarkana, in Hope, Ark., around the end of July.
A week before the discovery in Little River County, many growers in northeast Texas and some in southwest Arkansas “sprayed a combination fungicide product, a strobilurin plus a triazole. “Some fields in southwest Arkansas were sprayed twice. The second spray stopped it,” Coker said. “The pustules were there, but there was hardly any sporulation at all. So that's what's it's going to take. You're going to have to jump on it and stay with it.”
Coker noted that the southwest Arkansas soybean crop is mostly dryland, “but they had the rains and a good crop and were willing to spend the money.”
Later, temperatures rose to 105 degrees across much of the Mid-South. When southwest Arkansas growers asked about the need to spray again, “since there was no expectation for rainfall and temperatures were hot, there was no recommendation to spray. The weather was holding the rust down.”
In early September, about a million acres of Arkansas soybeans following wheat were still vulnerable to the disease, according to Coker. These late soybeans, many of them dryland, “have also gone through this 100-degree heat for several weeks and some of those fields are not going to cut 15 bushels. I can't see spending a penny on them. If the weather comes in, rust is going to build on those and provide inoculum for Arkansas and beans north of us.
“But there are some good fields where growers stayed on top of the crop and irrigated, and those could cut 45 bushels. On those beans, we're a long way from being out of the woods on soybean rust this year. We had a break from it with all the heat and high temperatures. But as this weather changes and it gets wetter, we're going to put out a call to protect our late beans. It may be a week or two down the road (from Aug. 31), a lot depends on how the weather develops and movement of ASR south of Arkansas.”
Coker says growers and county agents “will be on high alert for at least another three weeks. Our soybeans are maturing and we want those late beans to get into an R-6 growth stage, where they are touching in the pods.”
On Sept. 4, USDA reported soybean rust on soybeans in Washington County, Ark., and on Sept. 3 in a soybean research plot in Cook County, Ga. In 2007, rust has been reported in 10 counties in Alabama, five counties in Arkansas, 13 counties in Florida, eight counties in Georgia, 14 parishes in Louisiana, five counties in Mississippi, six counties in Oklahoma and 24 counties in Texas. USDA advises that “continue spread northward and eastward may be expected.”
Coker noted that some fungicides for rust have produced foliar burn, but studies indicate that this normally does not impact yield. “But a lot of the burn comes during hot, dry conditions, and from a rust management standpoint, we shouldn't be putting it out anyway. The dry weather really helps stop the disease.”
Coker added that spore traps aren't necessarily an accurate predictor of the presence of the disease. “In areas where we know that we've confirmed soybean rust, we don't always find spores. If we do find spores, we hit the area around it and check for rust. The best way to deal with it is to keep up with what is happening west and south of us and the current weather.”
One dubious, but positive, impact of Asian soybean rust has been the effect that fungicides have had on soybean yields, notwithstanding the actual presence of ASR. When you spray a strobilurin fungicide for rust, “you get the benefit of controlling other diseases. That's true even in Brazil. Rust is a tremendous problem there, but their overall yields are higher because they are protecting their beans from other foliar diseases.”
Cold winters normally will kill rust that moves into the northern Delta, Coker points out. “Rust has to have green living tissue — soybeans and kudzu. Soybeans are going to mature and be out of there. So it overwinters on small pockets of kudzu here and there along the Gulf Coast. But just because it was in those fields in southwest Arkansas this year, there is no way in the world to know if rust is going to be there next year. It cannot survive our winters here.”
Coker recommends that any time conditions are favorable for rust and rust is local, if you have good soybeans at R-5, I would strongly suggest that they be sprayed with a triazole. If you're early reproductive R-1 up through R-4 growth stages, full pod size and just starting to see the seed, we're going to recommend a tank mix of a strobilurin plus a triazole. To be safe from the disease, “your beans need to be at a solid R-6,” Coker said.
“It's a new disease for us to manage and we're on a steep learning curve and we're going to be conservative,” Coker said. “If we think there is a chance for a yield impact, we're going to recommend a spray.”