The start of the tropical storm season is only weeks away. So are damaging outbreaks of soybean rust just around the corner?
Spawned by tropical winds and moisture blowing up from Mexico, Texas and the Deep South, Asian soybean rust (ASR) has caused everything from near panic to complacency among growers since it was first discovered in Brazil and got American farmers worked up in 2004 and 2005.
In 2008, ASR was found in 392 counties across 16 states. It was confirmed as far north as Illinois. There were spots of damage, but no massive outbreaks or major yield losses. It could have been worse if not for a Texas drought that slowed development.
While winter weather killed most ASR in the U.S., in January rust was reported on soybeans in parts of Mexico. And in late February it continued to survive at relatively low levels on kudzu in a few southern states.
“A lot depends on the weather in April and May,” says Don Hershman, University of Kentucky plant pathologist.
“If it's wet and warm in the Deep South, rust could emerge and spread northward with high winds. A perfect storm would be a lot of rust overwintering on kudzu, then wet in the Deep South, then high incidence of rust in Mississippi, Alabama and other southern states in June on soybeans. That could lead to damage in the upper Midwest,” Hershman says.
That scenario has yet to happen, but doesn't mean growers can let apathy set in.
“We do know it won't be as common as we thought in the beginning,” says Hershman. “But to become complacent would be a mistake. There is still some active and overwintering in the South.”
The challenge over the past few seasons is that drought and high temperatures have set in to the southeast — conditions that are not favorable for ASR infection or development.
That's What Growers receive when they look at photos of ASR-infested fields. “It can be devastating. There is no other disease like it,” says Hershman. “It's a class by itself and merits special attention.”
Anne Dorrance, Ohio State University plant pathologist, says annual yield losses for soybean production could range from 10% to 90%. “If infestation occurs, they are predicted to be at least 10% in the upper Midwest, Northeast and Canada, and 50% or greater in the Mississippi Delta and southeastern states,” she says.
“However, losses in hard-hit areas anywhere in North America could exceed 80% if effective management tactics are not deployed.
“Be aware that in the early stages of infection, ASR looks very similar to many other soybean foliar diseases, including brown spot, bacterial blight, bacterial pustule, Cercospora leaf blight, downy mildew and frogeye leaf spot and abiotic factors such as burning herbicide compounds,” Dorrance warns.
“Few infections on soybeans are found prior to or at flowering under field conditions. It's more of a mid- to late-grain-fill disease,” she says.
ASR Can Form at temperatures between 68° and 75°. It needs only six hours to infect a plant, and generation is nine to 10 days under extended periods of cool, cloudy, wet weather and/or high humidity.
Weather conditions will not always favor widespread or severe rust development, even when spores are present. Hot, dry conditions will slow down or even stop disease development. Nonetheless, proper treatment is vital to prevent damage.
Hershman says strobilurin fungicides (azoxystrobin, pyraclostrobin, trifloxystrobin) can stop both spore germination and host penetration. “But these fungicides have little or no effect once the fungus has successfully penetrated or colonized host plant tissue,” he says.
Strobilurins include Headline or Quadris. Strobilurins plus triazoles (or propiconazole) include Quilt or Stratego. Fungicides containing only triazoles or propiconazole include Tilt, PropiMax, Alto, Punch, Topguard, Caramba, Laredo, Bumper, Folicur, Orius, Uppercut and Domark.
Strobilurins work best when applied before the disease hits a field. Only triazoles have curative activity. It is this postinfection activity that makes triazoles the fungicide of choice if ASR is established at low levels in a field, plant pathologists say. If chlorothalonil or one of the strobilurin fungicides is applied post-infection, existing infections will continue to develop, says Dorrance.
Triazoles have very limited curative activity, says Hershman, once infections begin to produce spores. That's one reason why fungicides are less effective once soybean rust has become even moderately established in a field.
David Lightfoot, Southern Illinois University plant pathologist, says that even though growers shouldn't be complacent about ASR, they should worry more about insects and other diseases in soybeans. “It won't be a chronic problem like that seen with sudden death syndrome, brown stem rot and phytophthora root rot,” says Lightfoot.
“They should concentrate on managing pests in proportion to last year's losses, more or less,” he says.
Some plant pathologists and growers are sold on regular fungicide applications, whether ASR is expected or not. “To apply fungicides without any specific target is a flip of the coin, a 50-50 proposition,” says Hershman. “There are a lot of pressures on farmers to spray.”
Lightfoot says fungicides used for ASR control benefit against other diseases. “There are lots of fungal sprays; each taking a bite out of yield, they add up,” he says. “I would recommend trying an insecticidal seed treatment, too, like southern growers are doing. It seems to help. Seedlings need protection.”
Even though treatment for ASR isn't needed after the R6 (full pod) stage, rust can still reproduce. In warmer southern areas, that means it can linger through the fall and winter.
Treatment for ASR may depend on yields expected from a field. If low yields are expected, growers may refrain from spending the $14-30+/acre likely needed to control it, says Hershman. Sunlight or ultraviolet rays will usually help control rust because it kills the spores either before they get to a field or after arrival, but before infection occurs.
Dorrance says this finding from Florida, Penn State and Iowa State researchers “is really having the biggest impact on our understanding of the slow movement of rust up from the south. Unlike corn and wheat rust, which move everyday with winds, soybean rust is only making huge jumps via rain storms as of today.”
Hershman says that even though some fungicides have label claims that they improve plant health, growers should acquire as much information as possible about a fungicide before counting on it as a cure-all. “In essence, we feel that this is going to encourage a lot of additional fungicide use for non-disease problems,” he warns.
Daily rust updates are available through the ipmPIPE soybean rust Web site at www.sbrusa.net.