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Five reasons why Asian soybean rust has (so far) fizzled

Agriculture.com Staff 12/21/2006 @ 12:59pm

Don't let down your Asian soybean rust guard yet.

But at this point, the odds of Asian soybean rust striking the Midwest are lower than researchers first believed two years ago.

There was good reason to fear Asian rust when it first surfaced in the United States in November 2004. "When you look at rust in Brazil, it's really bad," says X. B. Yang, an Iowa State University (ISU) plant pathologist who reported on his findings at this month's Integrated Crop Management Conference at ISU.

Left untreated, Asian rust has sliced Brazilian soybean yields up to 80%. Conditions conducive to the spread of rust -- leaf wetness for more than six hours a day and temperatures between 60 and 80 degrees -- are common during dewy evenings and mornings during the U.S. growing season.

Asian rust confirmation increased in the United States in 2006, as it was found in 15 states in a total 274 counties and parishes. However, massive rust infestations that many feared have not materialized.

So what gives? Yang cites the following five reasons why rust fears have not yet materialized.

1. Soak up the sun
Ultraviolet light zaps rust in its tracks. For example, University of Illinois researchers have shown if rust spores are exposed to sunlight for 48 hours, they will not germinate on host plants like soybeans.

Fortunately, the United States has lots of intense sunlight and sunny days compared to Brazil. "When you have a lot of lengthy cloudy days, the disease can be really bad," says Yang.

For example, Asian rust severity (level of damage) was 81% in the 2004-2005 growing season when 21 cloudy days per month occurred at an area in the major soybean growing province of Mato Grosso, Brazil. Severity dipped to 14% when eight cloudy days per month occurred in the same time frame in another soybean growing province, Rio Grande do Sul.

"Light intensity and duration increases as you move northward," says Yang. "We (in Iowa) have a lot more daylight in the summer compared to Brazil. So, light might play a very crucial role in limiting this particular disease."

That's something Yang noticed when he studied rust in kudzu patches in Florida in 2005. Kudzu is an alternative Asian rust host.

"The infection was only severe in the shade," says Yang. "Without shade, there was no or very little infection in the kudzu. It reminded me of what I saw when I was in Paraguay. Rust in the kudzu in the trees was bad, but there was no infection in the soybean field across the road."

Yang then decided to test four light intensity levels -- 20%, 50%, 70 to 80% and 100% -- upon rust fungus infection on soybeans. Findings confirmed observations Yang made from the Florida kudzu that high light intensity lowers rust spore survival and rust severity.

The effect of sunlight helps explain why rust first impacts the shaded, lower leaves of a soybean plant. It also relates to the way Asian rust originated in tropical Asia.

"In the jungle, you have plenty of shade, and soybean rust adapted to that," he says. "That's unlike corn and wheat rust, which developed in open growth."

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