With August's arrival, Asian soybean rust appeared to be newly invigorated — especially in the Southeast. Although innoculum levels remained low, the disease made the jump from sentinel plots to several commercial fields. And Extension specialists worried wet, humid conditions would allow the disease to spread further.
An overview of the disease since mid-July follows.
July 18 and 19
On July 18, Mississippi Extension specialists announced the discovery of soybean rust in a George County sentinel plot. The plot, in extreme southeast Mississippi, wasn't far from earlier rust findings in Baldwin County, Alabama.
Within 24 hours of Mississippi's announcement, Florida and Georgia had bad news of their own.
Researchers at the University of Georgia Coastal Plain Experiment Station in Tifton said rust had been found in a sentinel plot at the station. “Out of 100 leaves pulled, there was one leaf (from a plant at R-5) that had five or six pustules on it,” said Bob Kemerait, Georgia Extension plant pathologist.
Kemerait was troubled with the rust's location. “I thought if rust was found in Georgia, it would be in the southern part of the state: maybe Seminole County, coming out of Alabama. Tifton, in Tift County, is in the heart of the Coastal Plain, right on I-75 about 65 miles north of the Florida line.
“Unfortunately, if we've got rust here, I can't tell growers anywhere on the Coastal Plain they're safe from it. For that reason, I'm calling for growers in the area who have reached bloom stage, or beyond, to go ahead and spray a fungicide. I hate to break the news, but growers need to consider spraying. It's time.”
On the same day Mississippi and Georgia announced they'd found rust, a scout found the disease in an Escambia County sentinel plot in Florida's western Panhandle. Rust was confirmed the next day, July 19.
“Earlier this year, we set up sentinel plots (all planted in Group IIIs, Vs and VIIs) from Homestead all the way out to Escambia County,” said Jim Marois, Florida Extension plant pathologist. “The plot that came up positive is in the northwest corner of Escambia County. To give an idea of the area, Pensacola is in southern part of the county.”
Escambia County is also adjacent to Alabama's Baldwin County where rust was found in a sentinel plot and commercial field earlier in the month. “If you look at a map, the Mississippi rust site in George County isn't that far away either. There's a cluster along the coast where rust suddenly popped up.”
The Escambia plot was the second in Florida infected with rust. In early July, the disease was first discovered in Citra, south of Gainesville in Marion County.
More Asian soybean rust was confirmed on two sentinel plot leaves in Georgia's Colquitt County.
“The two infected leaves — out of 100 collected — were gathered…out of Group IIIs or IVs,” said Kemerait. “Those two leaves had pustules and an abundance of spores.”
At season's start, Kemerait said fears were of, “an explosion of rust. That hasn't materialized. We were preparing growers for an early, rapidly-developing epidemic.
“We're scratching our heads about why that hasn't happened. The most logical explanation is there's not a critical spore mass out there. Conditions have been favorable and the crop is at the right stage for rust. But speculation is there isn't a spore mass large enough to cause widespread disease.
“Of course, we don't know what the next few weeks will bring, but some of our beans are at R-3. Many are at R-2. We don't have to go that much longer before we're out of the worry window.”
The window had yet to close when Georgia next confirmed a brace of discoveries — in Brooks and Effingham counties — on the state's Coastal Plain.
“Finding rust in Brooks County isn't a big surprise — it's right on the Florida border, south of Colquitt County,” said Kemerait. “So now, if you look at a map, there's a line of counties with rust — Colquitt, Brooks and Tift.”
The Brooks County sample was collected July 29 on a private research farm. “One of the employees saw a suspicious leaf and brought it in. We check many suspicious samples that aren't rust. But this one was the real deal. There are some sharp people working on that farm.”
While not downplaying the Brooks County discovery, Kemerait described the Effingham County sentinel plot sample as “much more significant and disturbing.” The reason: Effingham County is just north of Savannah on the South Carolina border — well away from other areas rust has been confirmed.
“Now, in a broken line, from the Alabama border in the southwest corner of Georgia's Coastal Plain all the way up to the northeast corner of the plain we have rust sites. It's a light sprinkling, nothing major. But we have to assume it could be anywhere in the southern part of the state.”
Fungicide recommendations continued to be “any Coastal Plain producer with a crop in the R-1 stage or beyond needs to seriously consider spraying.”
Soybean rust was next confirmed in east-central Alabama on the Auburn University campus.
“We found rust in a sentinel plot near my office — about 100 yards from the lab,” said Ed Sikora, Extension plant pathologist at the university. “We collected samples from the plot July 29 and found three leaves on one plant that had about 20 pustules.”
In light of the new finding, “in central Alabama, farmers with soybeans beyond the R-1 stage should consider a fungicide application,” said Sikora. “That's definitely the case in south Alabama.”
The Auburn discovery is the “furthest north we've found rust this year. Even though it's of low intensity and there aren't any other reports outside Baldwin County, growers should put a fungicide out. Rust isn't moving as fast as we feared it would. But it is popping up here and there so growers need to keep an eye out.”
The rust's northern jump surprised Sikora.
“We've been monitoring sentinel plots in Headland, near Dothan in the southeast corner of the state. We've been checking those plots at least twice a week because it's near Seminole County, Georgia and some of the Florida positive sites. That area has seen a few instances of rust so I thought we'd see it there first. I keep thinking we'll find rust there but we never do. It's kind of odd that site remains clean and yet rust showed up at Auburn.”
On August 3, cases of Asian soybean rust were confirmed in east-central Alabama and southern Georgia. In both cases, plant pathologists said the disease appeared to be picking up its pace.
In Georgia, rust was again found in Tift County in a research field of Group VIIs that had begun dropping leaves.
“Development of the disease in Georgia hasn't spread over a huge area, but conditions have been right for it lately,” said Kemerait. “We're seeing rust respond more quickly than it has been.
“The rust has entered a corner of the plot and is causing defoliation in a limited area. But it seems to be spreading from there. The plants most heavily involved are infected from the plant top to the bottom.”
Asked if he expected more cases of rust in Georgia, Kemerait said, “Absolutely. It's conjecture, but with the weather we've had I expect it will be found in commercial fields very soon. The growth stage we're at and the widening distribution of the rust leads me to believe that. I feel comfortable with that prediction.”
Meanwhile, in Alabama, rust was found in an Elmore County research station sentinel plot halfway between Montgomery and Auburn. From Auburn, where rust was confirmed on Aug. 1, the Elmore County site is about 30 miles due west.
Sikora said the Elmore County plot was interesting because it was around 30 percent infected. “So from three leaves on a single plant last week, we're now at about one-in-three plants in this plot showing symptoms. It seems to be moving quicker.”
On Aug.5, for the first time, Asian soybean rust was found in a commercial soybean field in Florida. For the second time, it was found in a Mississippi sentinel plot.
The Mississippi plot, near Poplarville, was destroyed the day after rust was discovered. This followed a precedent established with the state's infected George County plot in mid-July.
“To keep any spores from getting to commercial fields that's the approach we're taking,” said Billy Moore, Mississippi Extension plant pathologist.
The Florida discovery was in Hamilton County, on the north-central border with Georgia.
“The rust isn't severe at all — from the central infection, the rust might have branched out a foot or two,” said David Wright, Florida Extension soybean specialist. “The soybean field it was found in was chest-high and bushy. The beans were Group Vs in the R-1/R-2 stage, planted mid-May. My understanding is a scout with the Florida Division of Plant Industry found it.”
As the crop moved toward maturity, Florida's growing conditions were excellent and patches of rust had been found across the state's Panhandle. “It wouldn't surprise me at all to find more rust…We're set up for more infection.”
For that reason, “we're recommending soybean producers in the early bloom stage use a fungicide with curative effects. If weather conditions remain this damp, another application may be needed in a couple of weeks.”
Wright asked producers to maintain good scouting. “Check five to 10 areas across a field because it's so difficult to find. Scout a different area of the field every time. And be prepared to use a fungicide.”
Also on Aug. 5, rust was confirmed at two new sites in Georgia's Laurens and Tift counties. Tift County has already had several incidences of rust. The rust in Laurens County is a first.
“In Laurens COunty, we found rust on a couple of leaves out of a 100-leaf sample on plants at R-5/r-6,” said Bob Kemerait, Georgia Extension plant pathologist.
“But even with that small amount, there was definitely pustules and sporulation happening.”
Rust has quickened its march across the state's Coastal Plain. “Laurens County is on the northern edge of the plain. This is further evidence Coastal Plain growers need to spray their crops with a fungicide.
“Laurens COunty is basically at the same latitude as rust sites in Effingham COunty and Auburn, Ala. Since that's the case, we're beginning to look harder at northern Georgia. Producers in north Georgia may not need to spray a fungicide immediately but they do need to be prepared to do so. I don't think it will be too long before we find rust moving into their area.”
The Tift County rust was found near Chuyla on a private research farm's beans at R-3. The sporulation from the site was much more pronounced than in Laurens County.
“Rust hasn't taken over the field, but there are a lot of leaves involved with many pustules. It's a bit more dramatic than what we've seen before.”
The latest cases have led Kemerait to believe rust has reached a critical phase. “I think we're now on the verge of an epidemic. Over the next few days, the weather is supposed to be rainy and I suspect the rust is about to take off for the first time this year.”
e-mail: [email protected]