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Corn+Soybean Digest

Be Prepared For Rust

As U.S. soybean growers are learning, Asian soybean rust is no respecter of crop quality. Regardless of the fertility, variety, land or herbicide program you use, your crop is fair game if rust spores land on it.

“You can do everything else right, but if you do rust wrong, it can be disastrous,” says Melvin Newman, Extension plant pathologist, University of Tennessee. “You've got to treat rust with respect.”

That means keeping a close eye on the disease this coming crop season and having a plan in place to address it if necessary, says Glen Hartman, USDA plant pathologist, based at the University of Illinois. Your best information resource, he says, is the USDA-developed Web site The site provides an in-depth, up-to-the-moment look at where rust is located on a county level, state by state. Plant pathologists across the country maintain the site by providing regular updates as the disease changes location and/or spreads.

Based on what pathologists learned in 2005, they are recommending two basic spraying strategies to address rust in 2006.

One strategy is based on preventing the disease, sometimes referred to as a plant health approach, which means spraying fungicides for rust prior to infection.

The second strategy is spraying for control, sometimes referred to as a curative approach, once rust is actually found in your fields. The word curative is somewhat misleading, however, as plant pathologists emphasize that none of the fungicides on the market are eradicants. Even the best fungicides will not kill all infection.

Whether you can benefit most from a prevention strategy or a control strategy depends largely on your location. Hartman explains that in the upper Midwest and further north, he recommends growers take the wait-and-see control approach to rust.

“We're saying don't spray until you actually see it, or you know it's just down the road in the neighbor's field,” Hartman says.

The main reason to wait is soybean foliar disease pressure tends to be lighter in the upper Midwest, where freezing temperatures in fall and winter will eliminate the presence and viability of rust spores. That means the disease spores must blow in each season to create another infection. You can, therefore, watch for the disease this spring and see if it's moving in your direction.

A second reason for the control approach: Fungicide activity is effective for only 14-21 days following application, says Don Hershman, University of Kentucky plant pathologist. So unless rust is in your fields or in close proximity, a preventative approach is not warranted and probably not in your best interest economically, he adds.

“You'll know whether the disease is coming your way in the Midwest,” he says. “There will be advance warning.”

However, starting in parts of the Midsouth and South, some growers may find they benefit economically from using a preventative spraying approach.

“We (in Tennessee) have more disease pressure and can spray and get a yield response from using a fungicide, even with a low infestation,” Newman says.

He says that the further south you go, where warmer temperatures prevail and foliar diseases can overwinter, the more true that becomes.

Newman adds that 50-85% of Tennessee farmers already spray fungicides annually for foliar diseases. “Our farmers aren't shaking in their boots about rust,” Newman says. “It's not a strange thing; it's just another disease to address.”

Triazoles and strobilurins are the two fungicide groups on the market most often associated with rust. Some of these products offer prevention only, while others offer both prevention and control.

The triazoles include products with myclobutanil, propiconazole, tebuconazole and tetraconazole. Triazole-based fungicides provide rapid systemic activity and are able to kill rust once it infects the plant. Some of the triazole active ingredients also can provide a measure of prevention.

On the other hand, plant pathologists say strobilurins appear to be most useful in preventative applications only. Strobilurin active ingredients include azoxystrobin, pyraclostrobin and trifloxystrobin.

For control purposes, Hartman recommends that growers either tankmix a triazole and a strobilurin together or opt for spraying a triazole alone. Hartman says research indicates that approximately 4 bu. of yield will pay for the product and application.

Because Asian soybean rust starts at the bottom of the soybean plant and moves up, it's critical to get excellent coverage at the bottom, Newman says. He tells Tennessee growers to wait several days following application and begin checking the underside of plant leaves for pustules. If they are present, he recommends that growers make a second application.

Prior to product use, pathologists say you need to determine whether the product you've selected offers prevention or control or both. Also check the label with your local retailer to determine whether your product is labeled for application via ground, air or chemigation. Fungicide labels vary considerably from product to product in this regard.

Pathologists say the critical time for the disease to affect soybean yield and quality are from flowering (R1), through pod formation (R4) and then at pod fill (R5). Once canopy is achieved, penetrating it well with a fungicide is next to impossible. “Early detection and treatment are critical,” Newman says.

However, rust is difficult to detect in its early stages, even for pathologists and consultants, Hershman says. He says the disease is not consistent in appearance and even a 10% incidence in soybeans is a challenge to confirm. “It's the hardest disease I know to detect at low levels, under field conditions,” he says. That's all the more reason, he adds, to keep track of where the disease is located on the USDA Web site.

Some soybean growers question whether other diseases or pests, such as aphids, can be addressed at the same time as rust, thereby saving the cost of applying another product and making another trip across the field.

“It's feasible, but it would be a fluke,” Newman says. Plus, he adds, some herbicides and fungicides are incompatible with each other in a tankmix.

His advice: “Spray just for rust, and get the job done right.”

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