New cases of Asian soybean rust continue to surface in Southeast

While still not in a gallop across the Southeast, the week of Aug. 22 saw Asian soybean rust move from a walk to a canter.

On Aug. 24, new soybean rust sites were confirmed in six Alabama counties: in soybean sentinel plots in Coffee and Henry counties; in commercial soybean fields in Houston, Dale and Pike counties; and on kudzu in Conecuh and Pike counties. Conecuh County is in south-central Alabama with all the other listed counties in the southeast.

“Infected beans in the sentinel plots and fields were at R-3 to R-6,” said Ed Sikora, Alabama Extension plant pathologist on Thursday afternoon. “Generally, the infection level seems low. However, I’m only walking a small percentage of the commercial fields. Some of these fields are 60 acres-plus in size. I believe incidence is ‘low to very low’ but could easily be missing hot spots within fields.”

In Georgia, rust continued its dogged northward trek.

On Aug. 22, soybean rust was confirmed for the first time in a commercial soybean field in Appling County at R-3/R-4. A day later, rust was found in sentinel plots at the Rural Development Center in Tifton.

On Aug. 24, new cases of soybean rust were confirmed in two research farm sentinel plots in Grady and Oconee counties. The Oconee County site, near Athens, is the farthest north that soybean rust has been found in the state.

On Aug. 26, a Putnam County sentinel plot — near Eatonton — was the next rust site confirmed. Pustules were found on 82 percent of the leaves collected from the plot’s Group II, III and IV plants. On the plot’s Group VII R-3 plants, 19 percent of the collected leaves were infected.

“The biggest surprise we’ve had is the way rust has progressed,” Bob Kemerait, Georgia Extension plant pathologist, said Friday morning. “It seems to be traveling around 60 to 70 miles per week. Once it got going, we were prepared for rust to be devastating. It hasn’t acted like that. It has spread across the state but hasn’t destroyed any fields.”

Both Sikora and Kemerait suspect the disease is more widespread than the sites found.

“It’s got to be in a lot more commercial fields than we know,” said Kemerait. “The rust spread has been so slow a lot of producers don’t even know it’s there.

“My feeling is we’re having little amounts of spores come in to start infection. That’s opposed to an overwhelming amount of spores moving in and infecting everything at once. In Brazil, there are huge spore showers that lead to rapid rust infections. Here, that isn’t happening.

“I’ll go so far as to say this: if we didn’t have sentinel plots and weren’t looking really hard for rust, I wouldn’t have guessed it was in the state.”

The disease remains difficult to detect in Alabama as well.

“When I’m out there checking for rust, I stumble across it — maybe because I’m more familiar with it,” said Sikora. “But it still isn’t at the point where you can pull up, get out of the truck and find it immediately at the edge of a field. I still have to get out and search hard. Finding it remains a mix of persistence and luck.”

How is Sikora picking commercial fields to check?

“In certain areas, every one I see, I’ll look at. We only have 160,000 soybean acres in Alabama so in some counties I try to get directions from Extension agents to areas with fields. In other cases, I’ll see soybeans while driving by and simply pull over and check them.”

Sikora is very curious about kudzu’s relationship with soybean rust. How chief a role the alternative host is playing in the rust’s spread is undetermined.

“There’s a tremendous amount of kudzu out there. Over the last two days, I was able to find rust in two kudzu patches out of a dozen I checked at random.

“Here’s the thing: I’m checking kudzu that’s accessible from the road. When you think about how much kudzu is off the road, though, it’s kind of overwhelming. There are acres upon acres of kudzu that’s going unchecked and could be acting as a host for rust.”

Sikora said it doesn’t appear soybean rust develops as rapidly on kudzu as it does on beans. “The pustules and sporalation appears to be much less on kudzu.”

Last year was supposedly the first for soybean rust in the United States. Assuming that timeline is correct, there were only a few months for soybean rust to over-winter in Florida.

“This year, we’ll have a full season worth of rust,” said Kemerait. “As a result, Florida’s kudzu may have loaded up with rust for the first time. That may mean a lot more innoculum starting off next year. If that’s the case, then this year we had perfect rust conditions but little innoculum. Next year, though, as rust continues to build in Florida and the Caribbean, we may see earlier and stronger infection.”

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