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Corn+Soybean Digest

Tricky Timing

While soybean specialists vary in their recommendations on how to address Asian soybean rust, they all agree that advance planning this spring will help you choose the option best suited to your specific needs.

If you're tired of hearing about the threat of Asian soybean rust, you aren't the only one, says Dean Malvick, University of Minnesota plant pathologist. At a rust informational meeting last fall, he asked the group of 150-plus participants if anyone was concerned about the disease occurring in their area.

“Not a single person raised their hand,” he says.

But now is a bad time for growers to turn complacent about the disease, say Malvick and other university soybean specialists.

“This March, disease levels were already at last year's levels reported in June,” explains X.B. Yang, Iowa State University (ISU) plant pathologist.

Because of mild temperatures, Yang says, Asian soybean rust over-wintered well in parts of some southern states and poses a more significant threat to soybean crops this season than it did in 2005.

For that reason, plant pathologists encourage growers to put a plan in place now to address the disease. One challenge for many growers, however, is determining whether to adopt a plan based on Asian soybean rust prevention, sometimes called the plant health approach; or, opting to treat the disease once it occurs in their fields.

However, Yang says a strategic control approach suited to your needs is even better, and that depends upon several factors: your fields' proximity to rust, the stage of crop growth if infection occurs, your soybean disease pressure in general, weather conditions and even your sensitivity to risk.

For instance, he says: “If rust is nearby, and it's early in the growing season, I'd recommend that you start spraying, especially if there's moisture and the weather is active. But if the disease doesn't show up (in your area) until late July, it's not going to cause much damage to the crop at that point, so you might opt to not spray.”

Yang says growers can stay abreast of the disease by tracking it via local sentinel plots and the USDA-developed Web site at

The most critical time for tracking the disease is from the R1 through the R6 growth stages, when the soybean crop is most vulnerable. During that approximate six-week period, your soybean crop will advance in formation from flowering to pod fill. Once your crop reaches R6, Yang says it faces little yield risk from rust.

If you decide a fungicide application is in order, consider that plant pathologists increasingly recommend that you spray a tankmix containing a triazole fungicide and a strobilurin fungicide. The combination offers both preventive and control measures, forming an effective offense and defense against rust.

Triazole-based fungicides provide systemic activity and are able to kill rust once it infects the plant. Strobilurins are useful in preventative applications only. One side note to bear in mind: No product currently on the market completely eradicates rust once it invades your field.

“I think most companies are coming around to the recommendation of using a tankmix of the two for best results,” says Marty Wiglesworth, Syngenta technical brand manager, fungicides. Along with improved control, he says using the two chemistries will help you minimize any potential for the development of resistance.

Cost-wise, a tankmix combination of a triazole and a strobilurin ranges between $10 and $16/acre. The triazole products, used alone, range in cost between $6 and $12/acre.

Wiglesworth adds that soybean plants within a field typically are not uniformly infected by rust when it moves in, so a single field can have more than one stage of the disease present. Fungicides are active for only about 14-21 days following application, so application timing is critical if your objective is strictly to prevent the targeted disease from infecting your crop.

One area of debate among soybean specialists concerns the preventive approach to rust control, sometimes called the plant health approach. In essence, while the fungicides prevent rust development, they simultaneously prevent or control soybean fungal diseases, including brown spot, frogeye leaf spot and stem anthracnose. The end result, Wiglesworth says, is that growers can reap additional soybean health benefits, such as reduced physiological stress to the plants, improved carbon and nitrogen fixation and increased moisture uptake, all of which contribute to improved seed bean size, quality and yield at harvest.

Soybean growers in the South and parts of the Midsouth achieve these benefits routinely because they typically experience heavier disease pressure in their crops. Midwest growers and plant pathologists aren't convinced the health benefit applies to them.

“Three-fourths of the time, even in northern growing areas, we'll see an economic return-on-investment,” Wiglesworth contends. He says Syngenta field trial results show an average 5 bu./acre yield boost when growers use the preventive or plant health strategy. The reason why, he adds, is because growers cannot always see a low-level presence of some of these diseases in their fields.

BASF also recommends a plant health strategy for fungicide use in soybeans. The company reports that fungicides can impact ethylene, a gaseous plant growth hormone found in soybeans. The fungicides reportedly cause a physiological response in the soybean crop, which helps delay crop maturity so the beans continue to grow and increase in size.

Martin Draper, South Dakota State University Extension plant pathologist, says summarized research across the northern states has shown a highly variable response in the upper Midwest from a plant health fungicide application, regardless of the product brand.“In one third of the locations, we saw a significant economic response of about 4 bu., just enough to pay for the fungicide application,” he says. “That wasn't the case in the other two thirds (of the locations).” Plant pathologists across the region are conducting research this season in an effort to determine those factors that contribute to the positive response.

Yang's perspective is that growers must compare treatment costs to potential return-on-investment to determine whether the plant health strategy is right for them.

Two upper Midwest researchers encourage growers to not use fungicides unless disease is a known threat to their crop. David Ragsdale, an entomologist at the University of Minnesota, found in 2005 that the same fungicides used to kill pathogens, such as rust, also kill all beneficial fungi, which help control damaging insects. As a result of fewer beneficial fungi, insects such as soybean aphids and mites were able to flourish in his university test plots.

Craig Grau, a plant pathologist at the University of Wisconsin, saw similar results last season for two-spotted spider mite populations in one of his soybean fungicide test plots.

Ragsdale says his hypothesis is that fungicides, if used indiscriminately, could negatively affect multi-year insect populations in soybeans, essentially increasing their prevalence. Bottom line, he says, growers need to maintain good IPM practices and use fungicides only when they know a disease problem is present.

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