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Corn+Soybean Digest

No Asian Soybean Rust Found, But Researchers Find Suspicious Spores

Louisiana Extension specialists have announced that three rust spores with features resembling P. pachyrhizi, the causal agent of Asian soybean rust, were found in a spore trap near St. Joseph, LA. While the spores have heightened the need for further scouting, no Asian rust has been found in fields.

“There’s no cause for panic,” says Clayton Hollier, Louisiana Extension plant pathologist. “We don’t even know if they’re Asian rust spores for sure. The fact the trap was next to a soybean field certainly makes it suspicious, though.”

In a Syngenta-sponsored project, traps – “essentially a glass slide with petroleum jelly on it,” according to Hollier – are being set up in many soybean-producing states. Regularly collected, the slides are sent for analysis to John Rupe, a plant pathologist at the University of Arkansas. Rupe found the suspicious Louisiana spores.

“My role is to evaluate the microscope slides coming out of the traps scattered around Louisiana, Arkansas, Mississippi, Kentucky, Alabama, Tennessee and southern Illinois,” says Rupe. “Others are evaluating trap slides from Georgia and other states.

“Eventually, there will be 100 spore traps and, ideally, we’ll be getting slides from them twice a week. Right now, most are sending the slides once a week. When we get a slide, we analyze it immediately.”

Rupe is “pretty sure” the three spores are from Asian soybean rust but can’t guarantee it. “At this point, with so few spores to check, we don’t have a technique that can definitively say. But unlike rust we’re familiar with – say on corn or wheat – these rust spores aren’t as darkly colored. Soybean rust spores also have particular spines or bumps on the surface. While other rust spores have some too, their ‘bumps’ tend to be much larger.”

Rupe has sent pictures of the three spores for USDA appraisal. “They agree these look like soybean rust spores.”

Hollier says the plant pathologist who collected the trap slide under scrutiny, Boyd Padgett, has scouted surrounding sentinel plots and soybean fields extensively. “He can’t find any rust there. One of the reasons we put this out is to make sure everyone knows as soon as we have any information, we give it out. We don’t want there to be any question about that. The last thing we want is for an outbreak to happen and then hear, ‘Why didn’t you tell us?’ That won’t happen.”

Hollier admitted it’s “kind of a no-win situation. Does releasing news about these three spores alarm some people? It probably does and that’s unfortunate. But, on balance, we believe it’s better to alert producers than take this complacently. Again: there is no evidence of rust in fields and sentinel plots are being looked at daily.”

On Wednesday, after releasing information on the three spores, Hollier visited a sentinel plot south of New Orleans. There, he collected many leaves and checked plants thoroughly but found no rust.

“There are other diseases out there. A lot of what I’ve seen lately is the beginnings of those, but not rust. We’ve got some cercospora – frogeye and purple seed stain – beginning to show up. Producers are treating for those in the southern part of the state. But that’s not abnormal.”

To do proper testing, to positively identify the spores, researchers say they may need hundreds. “We have three,” says Hollier. “These spores are microscopic. To see them with any detail, you need high power – perhaps as high as 400X. Growers have been told to use a 20X hand lens to scout plants. But that’s not going to let you see the spores, only the body the spores are produced in.”

And the usefulness of the traps hasn’t yet been determined. “This is an experimental technique,” says Rupe. “The truth is we don’t know how useful it will be as a practical method of predicting rust here. I’ve been told that by using the slides in Paraguay they were able to detect spores 10 days prior to seeing field symptoms. So it’s quite useful there.

“On the other hand, using the same type of traps in Argentina, spores were picked up well before the disease manifested – a month or two. So in Argentina, it wasn’t very helpful. It’ll take us some time to see how useful it is in the United States.”

Regardless, in much of the Midsouth conditions haven’t been conducive for Asian soybean rust to get started. David Lanclos, Louisiana Extension soybean specialist, who found out about the three spores on Tuesday afternoon, has been fielding “many calls on this. I’ve been telling producers and consultants who’ve been calling ‘Don’t get too excited. Potentially, these spores are floating around. But again – and we’ve been preaching this – the environment has to be right for soybean rust to take off. And currently, the environment just isn’t right for it.’”

As in much of the Midsouth, northeast Louisiana, where the spores were found, has been very dry. “A spore’s viability may last a week or a bit longer,” says Hollier. “But if it stays so hot and dry, the rust can’t get going. However, over the last couple of days, I’ve been on stations that have had rain and still haven’t found anything.”

Hollier says Padgett even has a misting system for part of his sentinel plots. “So there’s been plenty of moisture on those leaves and he’s still not finding rust. This may just be a flash in the pan. We certainly hope so and hope growers won’t be affected.”

The take-away message is two-fold, says Hollier. “First, the alert system is working. Second, don’t panic. Don’t pull the trigger on fungicides or practices you wouldn’t do ordinarily. Please, just continue to scout well and if you see something suspicious, call your local Extension agent immediately.”

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