Before summer 2013, soybean rust (SBR) had been relatively quiet throughout southern states, after threatening numerous fields in 2008 and 2009. Dry weather held down the disease that in 2004 had soybean farmers from Georgia to Minnesota scrambling to prepare for the yield-robbing fungus.
Clay Schaefer still takes no chances with SBR since it got within five miles of his east-central Arkansas farm in 2009.
SBR, first discovered in the continental U.S. in 2004, is a foliar disease caused by the fungus Phakopsora pachyrhizi. It can be aggressive, capable of causing defoliation and significant yield loss if infection and spread occur under conducive environmental conditions, says Tom Allen, Mississippi State University Extension plant pathologist.
Spores of the SBR fungus are transported readily by air currents and can be carried hundreds of miles in a relatively short period. Environmental conditions determine when and where the spores travel from south to north.
In 2009, SBR was widespread, and that’s when it nearly hit Schaefer’s farm. Some plant pathology labs were stacked with packets of SBR-infected leaves.
Scott Monfort, then a University of Arkansas Extension plant pathologist and now with Clemson University, said his lab was nearly overwhelmed — microscopes had little rest during that period. Hundreds of thousands of acres were sprayed with fungicide treatments to try and manage the fungus and limit yield losses.
Memories of SBR problems surfaced in summer 2013 when the disease was discovered in Arkansas, Alabama, South Carolina, Mississippi, Louisiana, Georgia and Florida — in some cases, earlier than in years past. A wetter summer and humidity created a good environment. Schaefer made sure he was ready for rust.
He farms about 4,600 acres near Hunter, Ark., half in soybeans, half in rice. This constitutes a good rotation that benefits both crops, he says.
The beans range from relative maturity Group 4.5 to Group 4.9. Some are no-till, some are bedded on 48-in. beds and some are conventionally tilled. All are 100 percent Roundup Ready.
Schaefer says he switched to earlier-maturing varieties in an attempt to reduce irrigation cost and get them finished earlier in September to beat the potential of rust in the fall. “I can’t go with earlier varieties because of my rice rotation, but I wish I could.”
Due to a cooler and wetter spring in 2013, he didn’t plant soybeans until the second week of June. In 2012, he was able to plant by the last week of April. The lateness of the 2013 crop had him more concerned about the potential threat from SBR.
Basically, if soybeans are pre-R6 development, they are generally considered to be threatened by SBR. That’s what Schaefer had faced before.
“When Scott Monfort talked about SBR at a local meeting in August 2009, we knew it was just south of us, five or six miles away. I had my consultant scout the Group 5s because the 4s were already beyond the R6 growth stage.
“I had already sprayed about one-third of the beans for prevention of anthracnose and aerial web blight. I had about 600 acres of later-planted Group 5s left standing unprotected, so I decided to spray with 5 oz. of a triazole. I can’t be sure that I stopped SBR — but I know I didn’t hurt myself by doing it.”
It was worth the $15 to $20 per acre cost for fungicide treatments to prevent SBR from damaging his crop, he says. “If I can save, or possibly add, even 1.5 bu. of yield, I can pay for a triazole spraying.”
No rust has made its way into his fields, but he says the triazole applications were ready to be made if needed. Other fungicide programs also help prevent diseases. “We have sprayed a lot of our beans with preventive fungicide treatments to cure frogeye leaf spot, aerial web blight and some lesser diseases,” he says.
“But, since frogeye has become resistant to strobilurin fungicides, I sprayed half of one field with Domark plus Quadris. I also applied Priaxor. We want to determine which program works best.
He uses crop consultant Jared Ford to help manage soybeans and rice. Ford was with him when SBR was a major threat.
“When beans were at R3, he came in to look for disease,” Schaefer says. “Because of pigweed resistance issues, he now looks at the whole crop, plus more acres, because I had less time to do the scouting myself.”
Mississippi State University Extension notes that SBR spores are disseminated by wind currents, then land on leaves. If environmental conditions are favorable, the spores germinate and directly penetrate plant tissue. Within six to seven days a pustule begins to develop.
Urediniospores begin forming and are released in an additional two to three days. Pustules can discharge spores over a period of two to three weeks, thus rapidly spreading SBR to other plants within a field or to distant soybean fields, depending upon wind currents.
Tom Allen, MSU Extension plant pathologist, says about 14 days is required from the time a spore lands on a leaf, leads to infection, produces a pustule and can begin to sporulate.
“Additionally, the majority of this will depend on the environmental conditions (temperature, cloud cover, relative humidity) at the time the spore lands on the leaf,” he says.
If SBR strikes, field losses in the 25 percent range are possible, especially in fields not treated with a fungicide.
“Preventive applications of effective fungicides, properly timed, will prevent yield losses due to SBR,” Allen says. “Producers should consider applying an R3/R4 fungicide application as a means of potentially preventing yield loss due to the disease.
“While a fungicide application at this particular growth stage will not provide protection for the rest of the growing season, it may delay infection from the SBR causal fungus.”
Higher soybean prices have helped Schaefer make the decision on whether to apply fungicides. “Soybean price helps in the decision whether to use a strobilurin or other fungicide early and come back with a triazole, if SBR or other diseases are threatening the crop,” says Schaefer.
For more on SBR symptoms and control, go to http://bit.ly/16zTepk or your state Extension soybean production sites.