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Early Asian soybean rust finds continue in Louisiana, Texas

New cases of Asian soybean rust have been found in Louisiana and Texas. Both findings are in kudzu patches in southern regions of the states: Louisiana's St. Mary's Parish and Texas' Liberty County, just east of Houston.

And the ASR has been discovered weeks earlier than in previous years.

“Our latest ASR is southeast of the first finding in Iberia Parish,” says David Lanclos, LSU AgCenter soybean specialist. “We're now in a rust threat situation — and I use that term conservatively. We have two early ASR locations, even though not on soybeans.”

In Texas, Tom Isakeit found the latest ASR in early June while touring kudzu sites in east Texas.

“I was looking at kudzu that had had ASR in the past,” says the Texas A&M plant pathologist. “And, as so often happens, the last place I looked is where I found the disease.”

Isakeit's latest ASR find was on kudzu under a bridge. Last fall, the patch — which spills from under the bridge onto both banks — had extensive ASR. Over the winter, the kudzu died back, “including the leaves out from under the bridge and I figured that was the last of the ASR there. When I found (the new case of) rust, it was confined to about 100 square feet and ranged from barely noticeable to extensive.”

Does Isakeit suspect the ASR somehow overwintered there?

“That is a major, important question because the kudzu is under a bridge and isn't really in a position to receive a bunch of spores. I would have expected to find any ASR on the exposed leaves unprotected by the bridge. But the ASR was in a protected area.

“Could spores and/or pustules survive a few months on seemingly dead vines and leaves? If so, that kind of throws a new factor into the ASR story.”

One thing is certain: this year, ASR has come calling two or three months earlier. Last year, it was August when Isakeit found ASR in a Liberty County kudzu patch.

What about fungicides? “I don't want to worry farmers, but if a fungicide is needed, it needs to be timed just right,” says Isakeit. “At this point, I'm still advocating a one-shot approach. This shouldn't be misinterpreted as urgency to spray a fungicide.”

In Texas' Liberty County, there's small soybean production — in the hundreds of acres. “The county Extension agent informs me the soybeans there range from newly-planted to seedlings. So the beans are too young to worry about ASR. And they may end up maturing during the hottest portion of the year when ASR has a tougher time spreading.”

The soybean crop to the south of the Texas ASR find is “well along and we've yet to find any evidence of ASR in sentinel plots in the area. Earlier this year, there was some ASR found in volunteer soybeans in Hidalgo County in extreme southern Texas, but that died out.

“I looked at some volunteer beans that have grown in that same field since. That was about three weeks ago and there was no indication of ASR at all.

“So there's a large soybean-growing area of the state along the coastline. Those are well beyond any risk. And if those beans did have an ASR epidemic, they could produce spores that could affect Arkansas or even further. But, so far, that area is rust-free.”

Asked about the early Texas ASR incidents, Lanclos says they're “another very interesting curveball 2007 has thrown us. If you look at where ASR was last year, Louisiana never had ASR to the west until late in the season. Back then, all the ASR concerns were from the east in Georgia, Alabama, Mississippi.”

While the Texas site is very small, “it's still producing spores that can blow into the Mid-South from the west. And Louisiana is definitely vulnerable from the west — to see that, just check the prevailing wind and weather patterns.”

Jeremy Ross, Arkansas Extension soybean specialist, says the ASR reports are yet to be “terribly worrying” for Arkansas. The Arkansas soybean crop — now 91 percent planted and 77 percent emerged — is getting dry and “we just don't have the conditions here for the spread of ASR. If it cools off a bit and we get more showers than we had in late May, it would be more concerning. Right now, though, we're waiting to see how quickly it progresses north.”

Lanclos says while no ASR has been found in Louisiana soybeans, questions about fungicide applications are picking up.

“We just finished an ASR meeting in southwest Louisiana where I said to try and postpone any application until R-3. At this point, I strongly suggest a triazole be added to the mix. There's no ASR, so it would be a preventive, prophylactic-type application. But with the price of triazoles, it's a safe, effective measure to prevent or control ASR in the most critical period to keep the beans stress-free — from R-3 to R-5.”

Lanclos says some producers close to the St. Mary and New Iberia ASR finds will opt for a two-shot fungicide plan. “I don't have an issue with that. Spraying twice is fine. But I'm basing my approach on economics and since ASR isn't in beans yet, I'm still promoting the single fungicide shot.

“The recommendation may change the day we find ASR in a sentinel plot or a production field, if a nearby field is post-R-3. But if your crop is prior to R-3 — and a lot of beans could be considering wheat-beans are still being planted — you probably need to be prepared mentally and financially to go with two shots. That's contingent on ASR moving into Louisiana beans, though.”

Just after the St. Mary Parish ASR location was confirmed, “I was very worried about having to go with two shots over a broad area. But after that we turned off extremely hot and dry. When the second location was pending, temperatures were still in the mid-80s with afternoon showers. Since then, temperatures have been 100 to 105 degrees and much of the state is very dry.”

Don't forget the other soybean diseases, warns Lanclos. “There are many more than ASR and R-3 comes around only once a crop. I regularly hear, ‘It's hot and dry and there's no disease, so I'm not treating.’ But I saw too many fields of producers with that attitude last year. It turned off very wet around R-4/R-5 and harvest was very difficult for some.

“I believe if you're going to grow a successful bean crop in Louisiana, you should strongly consider a strobilurin application at R-3.”

Back in Texas, Isakeit says he'd be more concerned with ASR if kudzu was more prominent across Texas. Right now, the invasive vine is “rather isolated. I checked out some kudzu to the north of this hotspot outside Livingston. It had ASR last year but is currently clean.

“And our conditions are getting hotter and drier. That tells me if there's overwintering rust in the kudzu vines, it'll be slow to become established.”

To Isakeit, the bigger question is whether the small kudzu patches are significant enough to trigger a major epidemic.

“I'm thinking the further you get away from this kudzu, the more diffused the ASR spores. So, in terms of a threat to Arkansas, I'd say this find is minor. In terms of a threat to someone growing soybeans nearby, or in the same county, that's a different story.”

Isakeit says his latest scouting has been at the southernmost limit for kudzu in Texas. “I've seen surveys that indicate the kudzu border appears to have a connection to the forested areas of east Texas. Those are higher in rainfall. But farther south, there's a slightly different climate and vegetation and there's no kudzu. And we've yet to find another overwintering legume host for ASR to take advantage of.

“If that holds up, it's a good thing. During most of the year, southern Texas seems free of any potential ASR host.”

(Editor's note: Shortly before publication, ASR was found in a commercial soybean field in Hidalgo County, Texas. Besides the Louisiana and Texas finds, as of June 17 ASR had been detected in 10 counties in Florida and five counties in both Georgia and Alabama.

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