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Corn+Soybean Digest

Rust Or Not

Alan Blaine “was not too concerned” about this summer's reports of Asian soybean rust (ASR) in Florida. But the Mississippi State agronomist wasn't writing off the scare.

“We treat everything as the real deal when it comes to reports of rust,” he says. If ASR does invade a field, Blaine's recommendation will depend on when you spray.

“Our data shows the best shot at controlling other diseases is at the R3 growth stage with a strobilurin fungicide,” he says. “In case of rust, a strobilurin and triazole would have both preventative and curative effects.”

Triazole alone or mixed with a strobilurin would be suggested for applications prior to R3 and if a second application is needed for ASR, he says.

But whether potential rust infestations are legit or not, Blaine encourages southern soybean growers to take advantage of the benefits provided by fungicides that are proven yield boosters.

Whether treating fields for ASR control or not, growers and researchers applying strobilurin fungicide have seen consistent yield increases near 6 bu./acre.

That same scenario is likely not the case in the Midwest, where yield increases appear insufficient to merit the cost of fungicide applications. “Don't spray if you don't have rust,” advises Anne Dorrance, Ohio State Extension plant pathologist.

The rust watch has been a bean-belt trend since ASR was discovered in South America a few years ago. The attitude of “not if, but when” has been the norm for many. Southern agronomists like Blaine have consistently monitored soybean sentinel plots and other field locations from Texas to Florida and northward. Those field checks were intensified last year when ASR was first reported.

Many chemical companies, their dealers and farmers stockpiled fungicide. But a dry 2005 prevented any major infestations.

Then there were early threats this year in south Texas. But rust didn't materialize, due to drought conditions there and in Louisiana, Mississippi and much more of the South. However, fears that Tropical Storm Alberto could bring new life rust conditions in the Southeast came true when University of Florida pathologists confirmed ASR in Group III beans in a south Florida sentinel plot.

Also, torrential rains on the Texas and Louisiana Gulf Coast had the watchful eye of Blaine and other agronomists, not to mention farmers in dire need of moisture.

Mid-summer rains in southern Louisiana and in Georgia likely brought about the two ASR reports in those states in early July. The soybeans were at R4-R5 growth stage, which researchers say supports last year's observations that more mature soybeans are more susceptible to rust than soybeans that have not flowered or are just flowering.

“Even though we're seeing the reports, it's the first sign of ASR this year,” says Blaine. “We have to bear in mind that it is such a small amount. Is it enough to cause a problem? I think the answer is no. I'm more worried about Texas and Mexico than Florida.”

Blaine encourages growers to exercise as much, or more, concern about more common diseases in their growing area, ones that can be lurking in the soil or air every year. Pod and stem blight can cause problems, as well as anthracnose, frogeye leaf spot, aerial blight or other fungus.

“We have a couple of diseases in the South that can make ASR look like a puppy dog,” says Blaine.

“Some growers forgot about those things and left money on the table in 2005. A good ‘strob’ program could have helped them,” Blaine says.

A program of strobilurin fungicide treatments has been instigated and monitored by Blaine since 1998. Out of nearly 140 trials, most in the pre-rust years, yields have averaged 5.9 bu./acre over typical yields.

“That is about 6 bu. over an average yield of 36-37 bu.,” says Blaine. “That's enough of an increase to pay for any rust program.”

If the fungicide application cost is $15-20/acre, and the extra 6 bu. add $35-40 more/acre in income (based on about a $6/bu. soybean price), that can add at least $15/acre to the bottom line.

Northern growers cannot expect the same returns, says Ohio State's Dorrance.

Dorrance says studies indicate that if fungicides are applied without a threat of rust, yields are increased on average by only 2-2.5 bu./acre.“It would cost more money than it's worth,” she says, referring to an application cost of over $15/acre.

The agronomists also remind growers that many fungicides that received a Section 18 temporary approval rating for ASR could easily be illegal to apply for other disease control. Labels should be read and followed carefully. In addition, quality spray adjuvants may be needed to assure proper plant coverage for fungicides and other chemicals.

Mark Wayland, marketing manager for value-added products, Helena Products Group, Memphis, TN, says the first place growers should look for rust-control advice is their regional Extension soybean specialist or agronomist. “They will be the ones who tell growers when to pull the trigger,” says Wayland.

“From our viewpoint, there are good products available to go in tankmixes to make those fungicides more effective in their protection and control of rust. Like Blaine, Wayland says there are many uses for fungicides other than for ASR control. “If products are in the barn from last year, research proves that (if they are labeled) there are reasons you may want to use them to control other diseases in soybeans,” he says. “We look for continued use of fungicides for soybeans outside of rust control, as well as with corn and wheat.”

Meanwhile, in mid-summer, Dorrance couldn't guarantee whether Ohio and other Midwest producers have dodged ASR again this year. She and others were still monitoring any measurable rainfall across the South that could spawn an outbreak and potentially send ASR northward.

She points out that Ohio State and other soybean breeding programs at many universities have been studying “slow-rust resistance” in soybean varieties. This trait would slow the development and spread of the disease and could be a step toward decreasing fungicide applications or eliminating fungicide treatments completely.

“Our fungicide program is only going to be a short-term solution to manage soybean rust,” says Dorrance. “If we're looking at managing the disease in this many soybean acres, then we're going to need host resistance.”

Blaine says, “Our southern climate is much closer to that of Argentina,” which has seen fewer ASR problems. “Nevertheless, we must continue monitoring for rust and other diseases that can damage our soybeans.”

(Editor's note: A list of EPA-approved fungicides for ASR control is in the April 2006 issue of The Corn And Soybean Digest and at For the latest information on ASR, subscribe to the Soybean E-Digest newsletter by going to Also, go to for national ASR information.)

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