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

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."

Yang says the ISU light findings are preliminary, and another year of data is needed to form firm conclusions. "But if it holds, early season outbreaks of rust are unlikely," he says. "Our findings are consistent with those of Illinois, that spores survive by lack of sunlight."

2. It rains more in Brazil
As initially thought, rainfall plays a big role in rust development. Rain falls in the rust-infested Mato Grosso province an average 15 to 20 days per month during the heart of its growing season in January, February and March. In comparison rain falls 8 days on average in June, July, and August in central Iowa.

Rainfall also is more prolific, averaging 8 to 10 inches per month in January, February, and March in Cuiaba, Mato Grosso’s capital. In central Iowa, growing season rainfall averages 4 inches per month during June, July and August.

"In Iowa, we have conditions suitable with rain for a disease outbreak once in every four years in June," says Yang.

The chances of a rust outbreak continue to decrease as summer progresses. In July, the incidence of rainfall in central Iowa favorable to rust outbreaks drops to once in every six to eight years. Even with the more favorable rainfall conditions in June, rust outbreaks dim due to the effect of sunlight on unclosed canopies.

"Even though conditions are favorable once in every four years from rainfall, the open canopy makes the disease harder to establish in June," says Yang.

3. It's higher in Brazil
Asian rust tends to be worse in high elevations where temperatures are cooler, Yang says. For example, many rust-infested areas in Brazil's Mato Grosso are 2,000 feet above sea level. When combined with the clouds and prolific rainfall, the higher elevation's cooler temperatures create a perfect storm for Asian rust to thrive. In the Gulf Coast states, early summer in 2005 and 2006 was hot and dry, which slowed disease build-up.

4. Wimpy U.S. wind currents
U.S. wind currents also discourage the spread of Asian rust spores. Initially, concerns existed about rust spores riding up to the Midwest on Texas air currents.

However, Yang says Texas is a mediocre spore producer. Spores that do form also have a short air current window that lasts just into spring.

"In May, winds can blow, but they veer off to the east," says Yang. "As the summer progress, there is a small chance of spores reaching Iowa."

Spores may also move into the Midwest from air currents from Florida via Mississippi, Louisiana, and Alabama. However, this window is narrow. "If they cannot move from Florida to the Mid-South and establish there before June, the risk for the rest of the season is not high," says Yang.

"If there's to be a massive outbreak, it likely will come on clouds spurred by a major weather event, much as what happened in 2004 with Hurricane Ivan," says Yang.

Even then, the traveling spores may not establish and proliferate in Iowa, due to sunlight intensity and rainfall patterns, says Yang.

5. Kudzu is a not-so-hot host
Before soybean rust arrived in the continental United States, kudzu was a big unknown factor as an alternative host.

"In the last two years, kudzu has been a so-so host in the U.S.," says Yang. "It cannot provide a load of spores before June. The second part of it is that kudzu likes sunshine, but soybean rust doesn't."

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

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