Could Friday's Asian soybean rust discovery in Iowa actually be a good thing?

Even with conditions perfect for the development and advancement of Asian soybean rust late this summer, the disease's first curtain call came too late in the growing season to inflict any crop damage. This, to Iowa State University (ISU) Extension soybean agronomist Palle Pedersen, is, in fact, a good thing.

Tests this week confirmed a small find of Asian soybean rust (ASR) in a soybean field in Dallas County, in central Iowa, officials announced Friday morning. The discovery marks the second time rust has been found in the state, the first coming last year on soybean residue found in a grain bin. That sample, which was later questioned by state officials, was confirmed as ASR in March.

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But, Friday's news of a new rust discovery in Iowa -- the nation's top soybean-growing state -- is no cause for worry in farmers, Pedersen says. The factors contributing to the northward advancement of the disease were strong in rust's favor leading up to harvest, yet upon arrival, the state's soybean crop is too far advanced to be affected.

"It takes very specific environmental conditions to get a rust epidemic. You have to have these right conditions -- if you don't have moisture and humidity, you won't get an outbreak," Pedersen says. "Look at how much rainfall we got this year in August. This has been a perfect year for rust. It just goes to show it probably won't be a problem ever in Iowa."

Wet conditions in August and September helped spur the development of rust at this time, says Pedersen. "It's been sitting in northeastern Oklahoma for the last two months and it really hasn't done anything until it surfaced in Kansas and this week in Iowa," he says.

It would be much more disconcerting is Asian rust surfaced during summer, when soybeans are in their critical reproductive phase. However, Pedersen reminds farmers that ultraviolet light via the sun can zap Asian rust spores. Since the soybean canopy is open a good share of the summer, sunlight would zap any spores in the making.

Rust prediction models created by ISU plant pathologist X. B. Yang indicated weeks ago that the chances were good that the disease would find its way into Iowa this year, Pedersen says. Officials knew it was on its way, but even in such a rust-conducive environment, it was and continues to be no reason for farmers to worry.

"[Yang] pretty much predicted four weeks ago that there's a more than 50% chance that it would come. It showed X. B.'s model was right on the money," Pedersen says. "We knew we had a pretty good chance it was going to show up, and when. And, this year was the perfect year -- we had lots of moisture and it was just right for the pathogen to explode. But, all the yields have already been made. Farmers have nothing to worry about."

This isn't to say growers shouldn't remain watchful for soybean diseases in general. Friday's confirmed rust case essentially places rust into the more general category of soybean disease for which farmers should watch throughout the growing season.

"There's nothing to worry about [with rust]. It would be better if everyone got as excited about finding SDS (sudden death syndrome)." Pedersen adds SDS is far more prevalent in Iowa than rust will ever be. It's more important that producers direct pest control efforts at pests like soybean aphids. Those who treated for soybean aphids last summer when they reached threshold levels are now reaping the benefits, with a number of yields averaging up to 65 bushels per acre in parts of Iowa.

"Most (Iowa) farmers are getting 55 bushel per acre yields than those who are getting 45 bushels per acre," says Pedersen. "We will be 90% done harvesting beans (in Iowa) in a week. Yields were made a long time ago."

Specific to rust, Friday's announcement is, in fact, "good news," Pedersen adds.

"The key thing for farmers to realize is that if it's going to show up, it's going to show up in a year like this," he says. "This is good news. This is a very good example that the system is working. In a year like this, when we know it will show up, it showed up this late.

"Farmers can relax."