Southern farmers got their first real taste of Asian soybean rust in 2005, and the consensus is that it could have been much worse, especially if they hadn't been prepared.
“I think spraying for rust at the R4 growth stage definitely helped our yields,” says George County, MS, producer S.L. Ferrill, who saw a rust outbreak in his fields at the end of the growing season. “Rust did impact our yields, but not to the extent it would have if we hadn't made a fungicide treatment earlier in the season.”
The rust found in Mississippi this past year apparently originated with tropical storms at the beginning of the summer, according to Billy Moore, Mississippi State University Extension plant pathologist emeritus.
“Tropical Storm Arlene infected kudzu plants in Florida between June 8 and June 10,” says Moore. “During that time, the storm moved rust spores into the Gulf, and a subsequent storm moved spores into Mississippi and Alabama, specifically George County, MS.”
The detection of rust triggered a fungicide application in the George County area, where most soybean plants were at the R3 or R4 growth stages. A recommended application of Headline SBR provided between 21 and 28 days of rust protection for those growers affected, says Moore.
He agrees that without a fungicide application, growers like Ferrill would have suffered substantial yield losses due to Asian soybean rust. “He had quite a bit of rust infection in his field, but he still made a reasonably good yield,” says Moore.
Another mid-summer storm resulted in Pearl River County sentinel plots testing positive for rust on Aug. 8, 2005, triggering fungicide recommendations for those soybean fields in the state south of I-20. “The good news is that we have since been unable to find Asian soybean rust on kudzu plants in that county,” he says.
Growers did the right thing last year, says Moore, adding that the situation could have been much worse. “There wasn't enough moisture for a long enough period of time to trigger a widespread outbreak of rust in Mississippi in 2005.”
Ferrill, who will plant the same soybean acreage this year as he did in 2005, isn't sure if a second spray for rust would have made a difference for his crop. “We'll spray this year depending on how things look at the time. I believe if I plant a little later — a Group VI variety — my yields might be better. The lack of continuous rain this past year probably helped us with rust,” he says.
Asian soybean rust was very much on the minds of Delta soybean producers in 2005, says Alan Blaine, Mississippi State University Extension soybean specialist.
“We got so focused on rust that some growers forgot to use fungicides for other yield-enhancing factors, such as the control of other diseases. Rust was the primary disease on people's minds in 2005,” says Blaine.
He says the focus on Asian soybean rust wasn't “hype.” Instead, it's a continuing threat. “But luckily we didn't have a problem with rust in the Midsouth. We didn't have rust at the R1 growth stage, so we didn't spray at bloom,” says Blaine.
Billy Wayne Sellers of Baxley, GA, says Asian soybean rust was late to arrive in his area in 2005. He kept a close eye on his fields and on an adjacent sentinel plot early in the season. When his Group VII soybean plants reached the R3 growth stage and remained rust free, Sellers sprayed a preventive treatment of the fungicide Stratego.
“Then rust hit us at the R5 growth stage. But because the fungicide we sprayed had a 21-day residual, we made the decision not to spray again,” he says. “The rust had very little effect on yield.”
Sellers gives credit to his early, preventative fungicide treatment for the negligible effect on yield. “We were hit in another area that wasn't sprayed, and we did have significant yield loss in that field.”
More than a month — about 45 days — lapsed between the date Sellers applied the fungicide and the date rust was diagnosed.
“We felt the residual in the spray would take care of it, and it did,” he says. “It was so late in the season, we didn't think the disease would do much damage anyway. At that stage, there is very little effect on yield. If rust doesn't hit you until R5 or later, you'll be in pretty good shape.”
As for his 2006 crop, Sellers says he'll stick with the same game plan as this past year. “We're going to do the same thing in 2006. We'll keep the sentinel plot on the farm, and if rust doesn't hit us by the R3 growth stage, we'll probably spray again with a fungicide that has some residual. Then, we'll pretty much play the rest of the season by ear.”
Sellers wasn't the only Georgia grower affected by Asian soybean rust in 2005, as the disease was widespread in the state, according to University of Georgia Extension agronomist Philip Jost. An initial find in late April on volunteer soybean plants in Seminole County didn't seem to have much impact on the crop. The major, sustained epidemic was first detected in mid-July in southwest Georgia.
“The disease continued to spread across the state and by the end of the season it had been found in counties bordering Florida, Alabama, South Carolina and Tennessee. We were never able to find the disease on soybeans in Elbert, Madison, Morgan or Hart counties, but it was likely hidden there somewhere,” says Jost.
Despite the movement of hurricanes and tropical storms out of Florida and the Caribbean, Georgia researchers were at a loss as to why rust did not become established earlier in the season.
“My personal belief is that the initial amount of spores being carried north to Georgia was quite low. The disease had to ‘percolate’ for a while at low levels on kudzu and soybeans in Georgia before reaching some critical level,” he says.
The direst predictions from Brazil had indicated that with the right environmental conditions, soybean rust could move 300 miles in a single day. But this didn't happen in Georgia in 2005, says Jost.
“As a very rough estimate, based on the amount of time it took to detect rust in Tift County and then in Oconee County — just south of Athens — we guesstimate that soybean rust was moving north in the state at approximately 60 miles per week,” he says.
The disease did not “explode” overnight in a field, he adds, although it may have appeared that way to some growers.
“We estimate that between 60 and 70% of the soybean acreage in the state was sprayed with a fungicide at least once to combat soybean rust. From field trials in Decatur, Tift and Appling counties, it appears that the R1 growth stage was an appropriate time to begin a fungicide program if there was indication from sentinel plots that soybean rust could be in the area. Delays in initiating fungicide applications led to reduced yields in several field trials,” says Jost.
Chlorothalonil was much less effective at controlling soybean rust than were other fungicides labeled for use against the disease, he says. “Unfortunately, we don't have sufficient data to say which fungicides were the best. However, we did see benefits from triazoles, strobilurins and triazole-strobilurin mixes.”
Jost says he is confident — based on his visits to growers throughout Georgia — that some producers who didn't use fungicides in 2005 suffered significant yield losses while other growers in the same general area who did treat weren't affected by rust.
“One of the most dramatic lessons from our fungicide trials was that treated rows of soybeans were not affected by untreated, completely defoliated rows next to them. Growers who treat their crop don't need to worry about the effect of neighboring fields that don't receive fungicide treatments,” he says.
While it's uncertain at this point how rust will affect Georgia's soybean crop in 2006, growers should expect the disease to be at least as bad as it was this past season, says Jost.
“Although we don't expect soybean rust to survive the winter in Georgia due to freezing temperatures, we do expect it to continue to build in Florida. We know that it can survive on kudzu in that state during the winter. We also found Asian soybean rust on Florida beggarweed in Attapulgus, GA, this past season, so the disease also may survive on this weed,” he says.