As of mid-June, Asian soybean rust hasn't been detected in the Mid-South. After finding rust in the Southeast earlier this spring, there were fears a wave of spores was on its way westward. Although no rust movement has been detected in the weeks since, vigilance continues.
“There are 38 sentinel plots around the state and about that many kudzu locations are scouted,” said Cliff Coker, Arkansas Extension plant pathologist. “We're scouting twice a week. If rust comes in, we want to know as early as possible.
“Right now, Asian soybean rust is in Florida and southwest Georgia. Those are the only locations it's been found. It's been in Georgia for around six weeks now. They've been checking closely, but it hasn't moved. Over the last couple of weeks, we've had some strong winds. Folks thought rust would be picked up somewhere else by now. That hasn't happened.”
Why no movement?
Many are interested in why the rust hasn't moved.
“One big reason has to be the environment,” said David Lanclos, Louisiana Extension soybean specialist, after consulting with plant pathologists. “With the exception of the last couple of weeks in certain parts of south-central Louisiana, things just haven't been conducive for rust. There has to be 10 to 12 hours of dew or moisture of some sort, and that's just not happening.”
Louisiana currently has a heat index hovering around 105. “That's above the optimum temperature for the rust to get going,” said Lanclos. “Now, I'm not saying it won't get going, but right now we're very hot. If you consider the conditions needed for this rust to spread, the Midwest should be more nervous.”
Following a recent tropical storm that hit Florida, Coker had many anxious phone calls. “Producers were worried this would carry rust to us. At this point, I don't think (Arkansas) will see anything from that storm. There are reasons I say that: we're dry and we're a little on the warm side. The weather isn't right for rust here.
“Also, it takes a tremendous spore load to be picked up and moved by a front. It hasn't even spread in Georgia. It just isn't a super threat to Arkansas at this time. It's a potential threat, of course, but I'm not overly concerned.”
A later arrival?
Will rust develop later in the year?
“It's possible,” Coker said at an Extension-sponsored producer meeting near Noble Lake, Ark., on June 14. “But it's also possible a hail storm will come through this evening and take out the cotton crop. I hope neither happens.”
Coker isn't overly worried about rust arriving in October when conditions are friendlier for it. “Our crop will be made by then. It's true we have some late-planted soybeans, and I'd be concerned about them. But, considering what we're seeing now out of rust, I'd be more concerned about frost damage.”
How late do producers have to worry about rust?
“An R-7 growth stage — perhaps a very late R-6 — is what you're looking for,” said Lanclos. “At that point, you don't need any more moisture, and the plant is drying down. Producers who're putting out a fungicide at R-3 must realize if rust comes in, they could still be looking at another application.”
Ignoring rust, Coker said if soybeans are at R-4 and the yield potential is good, a fungicide can be justified. “With a 50-bushel potential, I wouldn't hesitate to use Quadris or Headline. And that's spraying for diseases we normally have — I'm not even talking about rust.
“But as far as putting rust protection on beans currently: don't do it. It would be a waste of money because the best products we have for rust will do no good on diseases we normally have like frogeye, Cercospora, anthracnose and the rest. Instead, use that money for irrigation or save it in case we have to fight rust later in the ballgame.”
A fungicide will last about two weeks.
“Most growers have one fungicide bullet in the gun, maybe two if they stretch it,” said Coker. “So, at most, you're looking at two applications. That'll cost around $30. You don't want to spend any more than that on soybeans. So I'd keep those bullets in my pocket.
“If you need to use them later, we're likely to get plenty of word in advance. If rust is on the move, we'll know it. And just in case, we'll continue to scout our sentinel plots vigorously.”
Louisiana's earliest soybean crop is at the R-3/R-4 stage. Farmers there are still planting soybeans, primarily in the south-central region. In many locations, soils were too dry to plant early before heavy rains arrived.
“We're just now dry enough to plant in some places,” said Lanclos. “Thus far, I'm guessing we have planted about 725,000 acres.”
Thousands of soybean acres in Louisiana are being sprayed with fungicides. Spraying probably will continue through June. Lanclos said there are questions regarding exactly what's available to producers.
“Headline SBR — Headline plus Folicur — is being marketed only as a co-pack. So if a farmer wants to buy Headline alone, as opposed to Quadris, his only option is the co-pack. He'll have some rust protection because of the Folicur. Many producers will be using that because it's traditionally a bit cheaper than a Quadris/Topsin application.
“There is some concern in deep south-central Louisiana with Headline SBR specifically on Cercospora. I've spoken with growers and consultants who plan to put out Headline SBR now and follow it up with a tank-mix of insecticides spiked with Topsin at about a half-pound. I think that will be a common treatment.”
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