On July 18, Mississippi Extension specialists announced the discovery of Asian soybean rust in a sentinel plot in George County. The plot, in extreme southeast Mississippi, wasn’t far from earlier rust findings in Baldwin County, Ala. And within 24 hours two more rust discoveries were made — one in Florida, another in Georgia.
The isolation of the Mississippi plot (since destroyed to remove any chance it would send rust spores to commercial fields) heightened the need for scouting vigilance but didn’t warrant new fungicide spraying recommendations, state Extension specialists said.
“I found the rust in a plot of Group 5s in George County last Wednesday evening (July 13), late,” said Billy Moore, Mississippi Extension plant pathologist. “There aren’t many beans in George County. That remains the only spot in the state we’ve found any rust.”
What Moore found was “just symptoms, nothing obvious. I pull samples all the time based on ‘symptoms.’ Often, it’s just herbicide damage or something else. This time, it happened to be rust.”
Moore placed the samples in a plastic bag and that in a cooler. “There’s no ice in the cooler, the temperature stays 70 to 80 degrees. The sample stayed in the cooler until Friday night when I got back to (Starkville, Miss.) after a scouting trip. The samples were put in the fridge until they were looked at in the lab today.”
“As hard as he has been looking for this, I’m glad it was Billy who found it,” said Alan Blaine, state Extension soybean specialist. “It’s been harder than looking for a needle in a haystack. Even in this instance, he pulled 14 leaves and found rust on one.”
Both men cautioned producers against quick action with fungicides for rust alone. However, with other yield-sapping diseases like cercospora and frogeye showing up, producers shouldn’t assume fungicides aren’t needed at all.
“We can make money protecting our crop from things other than rust,” said Blaine. “Fungicides are good for maintaining soybean yields whether rust is around or not. Producers don’t need to forget that.”
Moore said his discovery involved “an extremely small amount of rust. It’s well away from soybean production areas. We monitor sentinel plots around Jackson, Ala., frequently. Rust hasn’t been found there. There are a few soybeans in the Meadville, Miss., area, a few beans near Poplarville, and a couple of other fields scattered around. Around there, beans are very, very sparse. There are so few bean producers there you can count them on two hands.
“The main news is this: we’re not concerned this poses a major threat to producers further north… Right now, there’s no reason for producers to be alarmed about this. No one should be losing sleep. We’re watching this closely. It isn’t a situation where there’s a wave of spores rolling north. There are so few spores being distributed right now. It’s like dropping a few grains of sand in the ocean.”
Shortly after Mississippi’s announcement, researchers at the University of Georgia Coastal Plain Experiment Station in Tifton said it was time for growers in their area to begin spraying. Asian soybean rust had been found in a sentinel plot at the station. Layla Sconyers, a plant pathology research associate, collected samples from the plot July 15.
“Because of the large number of samples, they’ve only been checked (July 18),” said Bob Kemerait, Georgia Extension plant pathologist. “Out of 100 leaves pulled, there was one leaf (from a plant at R-5) that had five or six pustules on it. Layla did a great job. She caught it very early in the infection.”
Rust hasn’t been found anywhere else in the state.
Further tests will be run on the sample. “However, there’s no doubt in my mind it’s rust,” said Kemerait.
The good news is the infestation was “very low. I went out to the plot tonight and there are a lot more bacterial pustules than anything that could be rust.”
Even so, Kemerait was troubled by where the rust was picked up. “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.”
Kemerait said conventional wisdom says the rust came in with Hurricane Dennis. The traditional rust development timeline doesn’t fit that, though.
“It was found last Friday. Dennis hit six days earlier. It’s hard to believe it had enough time to come in and get going in that amount of time. I guess it’s possible, but it would have had to hustle. Usually it takes well over a week for rust to reach that point.”
On the same day Mississippi and Georgia announced they’d found rust, a scout found the disease in a sentinel plot in Escambia County 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 3s, 5s and 7s) 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 has also been found in a sentinel plot and a commercial field. “If you look at a map, the Mississippi rust site in George County isn’t that far away. There’s a cluster along the coast where rust suddenly popped up.”
The Escambia plot is the second in Florida infected with rust. In early July, the disease was first discovered in Citra, south of Gainesville in Marion County. In that plot, rust was found on Group 3s at R-4/R-5. Marois said rust at the Citra site is now spreading to the Group 5s and 7s.
In Escambia County, rust was found on two Group 3 plants. “Rust was found on the second and third trifoliates with a lot of disease on them. There was also a Group 5 leaf hanging over the Group 3s that was infected.
“I think what’s happening at Citra — and we’ll likely see it at Escambia too — is as the inoculum builds, the Group 5s and 7s are becoming diseased too. Currently, since they aren’t as far along in the maturation process, they aren’t as susceptible as the Group 3s.”
The Escambia County sentinel plot, scouted weekly, is inside a 200-acre commercial soybean field planted about a month ago. The plot is of “substantial size and we found rust on only two plants. On the plants that came up positive I’d say the severity rate is ‘low to moderate.’ However, the leaves, as few as they are, had over 100 lesions on them.”
Florida Extension is recommending producers in the western panhandle counties — Escambia and Santa Rosa — consider spraying a fungicide as their plants come into bloom. Most soybeans in the state aren’t yet at bloom stage.
“They probably won’t be there for another 10 days or so,” said Marois. “That allows many producers a few days to get their ducks in a row.” Producers need to be on a “high alert” with scouting.
“Look carefully,” said Marois. “There isn’t enough profit in soybeans to spray a bunch of fungicides you don’t need. But if you find the disease in your field, you need to spray. Once this rust gets on the plants, it really takes off.”