Lower Spray Boom at Bloom

Rust discovery in Georgia means Southern growers need to be prepared to spray when soybeans start to flower. Pam Golden

Published on: May 16, 2005

During 2004, the question for soybean growers was what to do if Asian soybean rust reared its ugly spore.

The question coming in to 2005 was when rust would hit. That question was answered when rust was found on volunteer soybean plants in Seminole County, Ga. (the southwest corner of the state). The find was made by Georgia Extension researchers April 27 and confirmed by USDA on May 2.

That find answered the followup question: when to spray? The answer, in most cases for southern growers, is at first bloom.

Georgia Extension Specialist Bob Kemerait recommends growers spray at first bloom for three reasons: one, rust appeared early this year; two, spraying prior to flowering generally is not effective; and three, diagnosis is difficult.

“I now believe that it will be nearly impossible for growers to find the earliest stages of rust in their fields,” Kemerait says, noting that even USDA researchers spent days poring over the Seminole County sample.

“Diagnosis for us in Tifton has been at best difficult and often agonizing,” he says. “Because rust is now in the state and is difficult to detect early on, I believe that growers must be prepared to apply effective fungicides at first bloom, unless we do not find any spread of this disease in the future.”

Chances that the disease won’t spread are as likely as $10 soybean contracts.

“Models developed by the USDA suggest that spore deposition has already occurred this year over much of south Georgia,” Kemerait says.

With that in mind, Kemerait recommends growers take these steps:

  • Growers should anticipate spraying first bloom. Infection likely will have occurred by then and may be difficult to detect. He advises growers plan to apply a triazole fungicide for its curative activity. Tank-mixing a triazole fungicide with a protectant strobilurin will help to extend protective window on the order of one week (for example two weeks with a triazole alone, three weeks with the triazole/strobilurin mix).
  • Growers should plan to make a second fungicide application two to three weeks after the first application. This should be either a triazole or a triazole/strobilurin tank-mix.

In the triazole class, Kemerait says Folicur, Domark and Laredo have better efficacy than Tilt, Bumper or Propimax. Triazole/strobilurin tank-mixes/co-packs that should be available to growers include Headline SBR (Headline + Folicur), Quilt (Tilt + Quadris), and Stratego (Tilt + Trifloxystrobin).

Chlorothalonil products are less effective at controlling rust than triazoles or strobilurins, Kemerait says, though he notes that the chlorothalonil products are less expensive.

“Some growers may choose to use a chlorothalonil application when already making a ‘trip across the field,’ for example when applying boron or dimilin,” Kemerait says.

To track the soybean rust monitoring program in the United States, visit www.sbrusa.net