The year's first incidence of Asian soybean rust in a commercial field was discovered July 12 in southern Alabama's Baldwin County. The finding — in a 60-acre field of late Group 4s — occurred about a mile from a sentinel plot where the disease was found June 28.
“Overall, the incidence was at less than 20 percent. That's low severity,” said Ed Sikora, Extension plant pathologist with Auburn University on July 14.
About a week prior to picking up rust in the nearby sentinel plot, Extension specialists suggested the producer make a fungicide application. He did. Sikora suspects the disease “deposited in that field and the sentinel plot at about the same time. The symptoms we're seeing are a result of infection before spraying.”
The infected field will soon have a second triazole/strobilurin application.
The plant pathologist said soybean producers in the southern half of Alabama should spray a fungicide “if they're in the later reproductive stages: R-4 to R-5. In the area of the state where the rust was found (near Mobile), there's only one early-planted field — and that's exactly where the rust was found. The majority of the soybeans in the area are ahead of bloom. Last week, I was in fields there and most beans were in the second trifoliate stage, early vegetative. There's no reason to spray before bloom.
“Alabama growers should scout fields at least twice a week. If you're beyond bloom, have fungicides ready to go. At the R-3 stage, a fungicide application would be helpful even without rust. We're beginning to find other diseases. Yesterday, I saw frogeye, mildew, and what's likely cercospora. A fungicide, even in the absence of rust, will control these diseases and keep yields up.”
Alan Blaine, Mississippi Extension soybean specialist, agrees with Sikora. “Growers shouldn't focus on rust to the exclusion of plant health issues and other diseases. Where we have good yield potential, we make money spraying fungicides. All the Delta states have data supporting that. Many producers seem to be forgetting a strobilurin-based program helps yield. It's a money-making deal. ‘No rust, no spray,’ is the wrong approach.”
The week of July 18, Blaine said all Group 5s in the Mississippi Extension verification program will be sprayed with a fungicide. “We're not spraying them because of rust. We're spraying them just because it's time.”
The threat of Asian soybean rust has kept the South on its toes. As part of an early detection system for the rust, “spores of interest” were found in west Tennessee. A slide collected July 5 in a Jackson-area spore trap held two suspicious spores, a potential precursor to soybean rust.
“That means the spores landed on the (petroleum jelly-smeared) slide between collection dates — June 27 to July 5,” said Angela Thompson, Tennessee Extension soybean specialist. “Since it's such a small number of spores, a definite confirmation can't be made, just a visual affirmation that the spores look like soybean rust.”
In the same time period, 10 spores were found in a trap near Bowling Green, Ky., said Thompson.
“We've got 10 spore traps in Tennessee — one in Jackson at the experiment station where the spores were found. The main thing is we haven't found any presence or sign of disease on soybeans. And that includes the sentinel plot the soybean trap was located in.”
For the next several weeks, Thompson said, extra samples will be collected from the Jackson sentinel plot. “We'll test for rust. We'll also be stepping up spore trap slide collections — now, at least twice a week. That's not only due to the spore find, but to make sure we're on top of anything the big storm from the Gulf brought our way.”
Tennessee's growing conditions have been very similar to other dry states in the Mid-South. Many areas have had very little rain, resulting in short soybeans.
“It doesn't excite producers at all to think about having to spray a fungicide on some of the little beans. Overall, the dry conditions weren't conducive to disease development. Even if we did have two soybean rust spores, I don't believe the conditions a week or two ago would have promoted the disease.”
Recent rains may change that. “Today, temperatures are in the 70s and it's humid. Everything has gotten a good soaking. So conditions are better for rust. But I don't know if we've got a big enough spore load in our area to cause problems. At this point, there's no reason to be extremely worried. We'll continue watching this closely.”
The Mississippi soybean crop has great yield potential. When an irrigated field hits R-3, “I say hit it with a fungicide,” said Blaine. “I'm going to hit my beans like there's no rust out there. The worst-case scenario is I spray the field with a strobilurin. Then, if rust does show up, I can come back with a triazole. Using that approach, a grower is potentially cost another trip in the airplane. And, by that second spraying, we may need to treat for stinkbugs anyway — just add to the mix.”
Billy Moore, Mississippi Extension plant pathologist emeritus, is involved in scouting/planning for Asian soybean rust in the state. “Since yesterday, I've put over 700 miles on my vehicle,” said Moore on July 14. “We're looking hard for this disease.”
Over the last few years, Mississippi has been the earliest planted soybean state in the country. Currently, most of the state's crop — “maybe 70 percent,” said Moore — is past R-3, closer to R-4.
“The one factor that concerns me is the weather conditions have become more favorable for rust. I've been in and out of showers all day long and it's cooled down a little.
“However, right now, the amount of rust spores available is extremely low. And the air current isn't blowing them into Mississippi. There's been one exception to that: about two days before Dennis moved in, the weather modeling folks said rust could have moved into southeast Mississippi. Beyond those two days, though, nothing of much concern.”
Blaine echoed Moore's beliefs. “As of 8 p.m. on July 14, we haven't found any rust in Mississippi. It may show up tomorrow. It won't surprise me if we find rust in the next 10 days. Chances are Dennis brought something here. But the next question you have to ask is: how much did it bring?”
Given how far along Mississippi's crop is and having watched the progression of rust since it was found in Florida in mid-February, “I don't think we've got much to worry about,” said Blaine. “I don't see it building to massive levels, particularly since it hasn't moved very far north.”
Some Mississippi soybeans are “done today. In three weeks, half our crop will be done at R-6. Any rust that comes in after that will have minimal impact. By Aug. 15 (the vast majority) of the crop will be ready.”
“At this point, we're not suggesting a blanket treatment for rust,” said David Lanclos, Louisiana Extension soybean specialist, on July 18. “If a farmer feels it's necessary to add something to the tank for rust, fine. We don't think it's necessary but that's not our call, ultimately.”
Like Blaine, Lanclos said producers don't need to forget the diseases that are being seen. “Aerial blight, frogeye and other things are being picked up down here. These things are coming on slowly but surely.”
Lanclos said some 60 percent of Louisiana's soybeans are “around a week” from being out of danger from soybean rust. The remaining 40 percent of the crop — primarily in the southwest and south-central regions of the state — is still a month from outgrowing rust fears.
In Arkansas, the earliest planted beans are already out of danger, said Rick Cartwright, Extension plant pathologist. However, at least a third of the crop was planted in June and July and remains a concern.
“We're not out of the woods,” said Cartwright on July 18. “We've found no rust yet but that doesn't mean we can relax our guard.”
“I continue to be impressed with producers' calmness regarding this,” said Lanclos. “They're educating themselves well because with low commodity prices no one wants to spend money frivolously.”
With rumors and recommendations to spray for soybean rust coming from many directions, Blaine said producers often hear conflicting advice. “The producer has to make the final call. But if he hears something that doesn't sound quite right, he should call us. There are some wild recommendations being made. Don't believe everything you hear. Before spraying for rust, make sure you've heard the full story.”
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