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IF THE PROSPECT of Asian soybean rust (ASR) hasn't already given you a scare, keeping track of the treatment options could add to the fright.

A dozen or more fungicide brands are likely to be approved for ASR control before treatment season rolls around. Unless you grow wheat or other crops where fungicides are common, most of the brands will be unfamiliar.

But there's less here than meets the eye. The accompanying scorecard (page 74) sorts out fungicides by chemistry families and treatment strategies. It simplifies the task of getting to know the world of ASR fungicides. Sure, there are differences between active ingredients within chemistry families, but understanding the families and how they fit into treatment strategies will help you decide what products to use if ASR strikes your acres in 2005.

Three fungicide families

Major ASR fungicides fall into three families: chlorothalonil (sometimes called nitriles), strobilurins and triazoles (also called sterol inhibitors).

The two treatment strategies are preventive (treat before the ASR fungus is present) and curative (treat after the fungus has arrived). These strategies dictate which fungicide families to use.

Both chlorothalonil and the strobilurins are preventive fungicides; they have little or no efficacy on ASR if it already is present on soybeans when applied. The triazoles, on the other hand, are both preventive and curative. In addition to preventing infection when applied before ASR is present, they also control ASR if applied when plants are already infected. To be effective as a curative treatment, however, they must be applied in the early stages of the disease, when it can be hard to identify. If application is delayed too long, ASR already may have severely reduced yields.

Those are the basics, and on the surface, they are pretty simple. But in reality, devising your own treatment strategy is complicated by details like being able to positively identify ASR, making sure the right product is available when you need it and having spraying resources available at the right time.

Preventive or curative?

The first step in devising a treatment strategy is to decide whether to use a strictly preventive or early curative treatment program, or some of each. Either program can be effective, says Alison Robertson, extension plant disease specialist at Iowa State University.

“I don't recommend one treatment strategy over the other,” she says. “We really don't know enough to say one program works better than the other. It all boils down to the approach the farmer is most comfortable with.”

Preventive programs may be better suited to conservative, risk-averse growers, she says. By treating before the disease strikes, you can avoid significant yield losses. The downside: You're committed to a $15 to $20/acre treatment before you know whether it is necessary.

Curative programs entail higher risk, because to effectively protect yield, the fungicide must be applied early in the disease — either before symptoms appear or up to 10% disease incidence in the lower canopy. “When there is more than 10% incidence in the middle of the canopy, a fungicide application may not be cost-effective,” Robertson says.

The challenge is to identify ASR early — no easy task. “For someone who is not used to identifying soybean diseases, it can be very difficult to identify ASR versus other diseases that might be present,” she says. Her advice is to educate yourself about ASR this winter, including scouting techniques. Outside experts will be able to help, but the more you know, the better.

Wayne Pedersen, a soybean pathologist at the University of Illinois, recommends that Midwest soybean growers use a preventive program on at least some acres if ASR is identified in monitoring plots in the southern U.S. during the 2005 growing season.

Even if ASR doesn't strike, a preventive fungicide treatment probably will at least break even, he says. “Over the past five years, when we have sprayed a strobilurin at the R3 stage (early pod set) in research plots, our average across-the-board yield increase is 4 to 6 bu./acre,” he adds. Yield response is due to control of diseases such as Septoria leaf spot, anthracnose, and pod and stem blight, which are common in soybeans and reduce yields in many parts of the Midwest.

With an estimated $15 to $20/acre spraying cost, preventive therapy probably will at least pay for itself, with ASR control as a bonus if it strikes, Pedersen says.

Treatment recommendations

Along with soybean groups, universities and agricultural retailers, ASR fungicide manufacturers are working this winter to educate growers on proper use of their products. Uniformly, manufacturers emphasize that early application is critical, whether a preventive or curative strategy is used.

Several manufacturers recommend using a mix of a strobilurin and a triazole applied early, before ASR is present. Both premixes, as well as tank mixtures, are possibilities.

“Ideally, you want the application to be preventative,” says Allison Tally, technical brand manager for fungicides for Syngenta Crop Protection. “The triazole part of the mix has good curative activity. It gets into the leaf quickly and stops the disease from progressing if it happens to be present. The strobilurin has long residual activity and provides broad-spectrum activity on other diseases.”

A triazole fungicide alone also can be an effective preventive treatment, providing up to 21 days of residual activity, says Bob Gordon, Dow AgroSciences senior market specialist for fungicides. “The optimum application is always made before the infection occurs,” he says. Because symptoms typically aren't apparent for 7 to 10 days after a field is infested, it can be difficult to know whether the disease is present when an early application is made. That is why a triazole's curative and preventive capabilities in the first spray are important, Gordon says.

“Once symptoms are visible, you may have only five or six days where you have any opportunity to stop the disease and prevent yield loss,” he adds. “If everybody waits to apply until after they see symptoms, there may not be enough sprayers available to make applications in a timely fashion.”

Paying attention to sentinel plots planted across the South and the Midwest will be an important aid in application timing, says John Smith, fungicide business unit manager for Bayer CropScience. “Because this disease is very aggressive and travels rapidly on prevailing winds, knowing what is happening to the south will help identify the best application timing.”

Universities, manufacturers and agricultural publications have established Web sites to convey information about ASR. Visiting these Web sites during the growing season will help you track progress of the disease from south to north.

Asian soybean rust fungicide scorecard*
Bravo WeatherStik Chlorothalonil Preventive Syngenta Crop Protection
Echo 720 Chlorothalonil Preventive Sipcam Agro
Bumper Triazole Preventive/Curative Makhteshim-Agan
Domark Triazole Preventive/Curative Valent U.S.A.
Folicur Triazole Preventive/Curative Bayer CropScience
Laredo Triazole Preventive/Curative Dow AgroSciences
PropiMax Triazole Preventive/Curative Dow AgroSciences
Tilt Triazole Preventive/Curative Syngenta Crop Protection
Headline Strobilurin Preventive BASF
Quadris Strobilurin Preventive Syngenta Crop Protection
Stratego Triazole + Strobilurin Preventive Bayer CropScience
Quilt Triazole + Strobilurin Preventive Syngenta Crop Protection
*Use registrations for some fungicides are pending. Check with state regulatory officials for fungicides approved for use in your state.
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