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Fungicides key to Asian rust control

Fungicides will play a vital role in combating Asian soybean rust. But before dealing with Asian soybean rust, said Dan Poston, it's important to know what benefits fungicides provide in its absence.

Towards this goal, the Mississippi Extension soybean researcher presented an overview of foliar fungicide work at the Tri-State Soybean Forum in Dumas, Ark.

“Some growers are saying, ‘We can't afford to add another $30 to our production budgets.’ What many don't realize is there may be some fungicide benefits beyond what they expect,” said Poston, who is based in Stoneville, Miss. “In essence, there may be a way to pay for part of the rust program that isn't obvious… If you've never used them before, you may be surprised at some of the benefits you'll get from using a fungicide.”

Higher yields?

Poston points to fungicide usage in the Mid-South in the 1970s and 1980s. Results were sometimes inconsistent with only moderate yield increases. At that time, soybean yield potential was “pretty low” region-wide. Fifty bushels was an excellent yield.

“Now, irrigated beans not hitting 60 to 80 bushels are a disappointment for many producers.

“Back then, we had some things like a fungicide point system to help us. Based on several parameters, the system told you when to spray and 90 percent of the time, the system was dead-on. The problem came when it said not to spray — it was correct only half the time.”

Fungicides eventually came into major use by seed producers to improve quality. However, fungicides were rarely used by strict oilseed producers. Now, things have changed.

“We've moved into earlier production, we have higher yield potential and we've shifted to indeterminate varieties. Several years ago, strobilurin fungicides — like Quadris and Headline — came on line offering broad-spectrum, relatively inexpensive control of diseases, like aerial blight, that we couldn't control before.

“In Mississippi, with the arrival of these new fungicides we saw a great need to reevaluate foliar fungicides and how they relate to soybean production. Most of what we studied were Quadris-based programs. Over the last two years, we've conducted experiments in nine locations. Both R-3 and R-5 tests have been conducted at each location.”

Poston said researchers looked at 12 treatment programs. Along with new fungicides, “we also included some older products, like Topsin M. Averaged over all locations, a lot of products — including 6.2 ounces of Quadris, reduced rates of Quadris (either alone or mixed with Dimilin, Topsin M, or Tilt) and Topsin M — all significantly increased soybean yield. Bravo produced a small yield increase, but it didn't carry the weight the others did. In terms of yield response, Tilt alone didn't bring much to the table.”

In photographs of this year's Leland, Miss., test location (near Stoneville in northwest Mississippi) areas of the field are bright green alongside dark spots scattered throughout. The darker areas were either untreated or spots where the foliar fungicide didn't work.

The producer's untreated, irrigated beans — Asgrow 4603 planted around March 20 — yielded about 65 bushels. Poston's untreated check yielded the same and, he said, “the producer was fairly satisfied with that number — at least until he found out what the treated plots yielded.

The plots treated at R-4 with 6.2 ounces of Quadris alone or mixed with other fungicides cut 82 bushels. The same treatments increased yield about 8 bushels per acre when applications were made at R-5.”

The numbers

Yield bumps are welcome. The problem is “just because we see yield response doesn't mean much. We have to know if these programs will pay for themselves. Looking at net returns, we ended up with an average selling price of $6.09. Across all programs we tested, the 6.2 ounces of Quadris alone or mixed with Dimilin were the only ones that statistically increased net returns, but the trend was towards increased net returns with several fungicide programs.”

The tests did include some low-rate programs that did well. However, “let me emphasize, these responses were seen prior to Asian soybean rust. I don't think low rates of strobilurin chemistries will be an option now. In light of the pathogen we're dealing with, using low rates probably isn't a good idea — at least for this year.”

With much data before the researchers, “we asked ourselves, ‘If I sprayed my $5 beans, what is my likelihood of getting a positive economic return?’”

The answer hinges on bean prices. There's a 60 percent chance of getting a good economic return on $5 beans. If the bean price moves to $6 or $7, the chance the economic outlook brightens even more.

“The price has a lot to do with the likelihood of a producer getting a positive economic response from foliar fungicides in the absence of rust.”

There's a problem with averages, though, said Poston. “You may end up with a 16-bushel yield bump or you may end up costing yourself money by using a fungicide. What we want to know is how to increase the likelihood for a large yield response.”

On the test plots, half the applications went out at R3-R4 and half to later growth stages. Looking at all the responses that varied due to timing, treatments put out at R3-R4 averaged about 62.7 bushels. The applications at R5-R6 averaged only 60.2 bushels.

“So the R3-R4 treatments were 2.5 bushels better than later sprayings.

“Other states have seen similar yield responses. Kentucky has shown a 4.4-bushel response to Quadris mixed with Warrior. Illinois and Indiana saw yield jumps of around 5 bushels. Those numbers are similar to what we're seeing here, but they don't appear to have the overall disease pressure that we have in the South.”

Of course, Poston and colleagues want to increase the incidence of large yield responses. In Kentucky, 26 percent of the time they saw very minimal responses. In Mississippi, the numbers are even worse: 35 percent of the time, tests showed less than a 2 bushel response.

“But I see that a different way: 65 percent of the time, we're seeing more than a 2 bushel response! You have to look at the big picture.”

Is everything rosy with foliar fungicides? “No, there will be problems. In some locations, we see a greening effect on soybeans. In some cases, with the right variety in the right environmental conditions, you'll see a lot of green stems that might cause some harvesting difficulties.”

If a producer elects to use a strobilurin-based fungicide program, “he'll likely increase yields across his farm,” said Poston. “That will help pay for some and, in large yield increases, perhaps all his rust treatments. Strobilurins aren't particularly cheap, though. If you're going to get the most bang for your bucks in a rust management program, you probably need to look at an R3-R4 timing with strobilurin fungicides like Quadris and Headline.”


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