Rust Risk

Farm Journal, Feb 1, 2008 by Pam Henderson

Soybean rust is a reality. Just ask Billy Wayne Sellers. The fungal disease hit his Baxley, Ga., soybean fields in 2005 and has been back to visit every summer since.

"I'd never sprayed a fungicide until rust came along," Sellers says. Now he makes a routine fungicide application in the early reproductive stages (R1 through R3) and then comes back two to three weeks later as needed.

An estimated 1.34 million acres were sprayed for soybean rust this past season. Louisiana growers treated 85% of their soybean acres prior to the R6 growth stage, while Georgia growers sprayed 75%; Alabama, 15%; and Mississippi, 5%.

Weather factors limited what might have happened in the southeastern U.S last year, explains Carl Bradley, University of Illinois plant pathologist. "Rust outbreaks are building each year, and it is being found earlier each year," he says.

"What the 2007 soybean season did was dispel doubts the disease has the potential to reach across the Soybean Belt," Bradley adds.

The confirmed outbreaks of rust in Northern regions came late enough in the season (R5 and beyond) that yields were already made and fungicide treatments were not needed.

In 2007, soybean rust was positively identified in 19 states and 339 counties in the continental U.S. The disease also made appearances in Hawaii, one province in Canada and two states in Mexico.

Rust has already been found overwintering in kudzu this year. The likelihood of increased soybean plantings in Southern states means there will be more of the preferred host available.

On alert. At the University of Kentucky, plant pathologist Don Hershman says farmers should not fall asleep at the wheel.

"The reality is we begin to catch spores of the rust fungus in Northern states by midsummer," Hershman says. "So far, those spores have mostly been dead on arrival. Plus the numbers have been low and conditions generally unfavorable for infection.

"But given the proper [moist] conditions in the South [where rust is sure to overwinter] in April through June, it's not hard to imagine a scenario where soybean rust could move into the Soybean Belt at a critical time in the season," he adds.

In June 2005, Tropical Storm Dennis moved from the Gulf Coast states into Canada within a 36-hour time frame. "At that time, there were not enough spores present to spread soybean rust," Hershman says. "But that's the kind of storm and the time of year when a serious situation could arise following a wet April to June in the South."

The key for farmers is to realize the threat and to track the disease and its severity. An easy and accurate way to track rust movement is to monitor USDA's pest management Web site at www.sbrusa.net.

Soybean Rust's Triple Threat

Three critical factors affect the spread of Asian soybean rust.

Overwintering: Soybean rust must survive winters in southern North America on susceptible plants that keep their leaves. Kudzu is the prime alternate host.

Spore movement: The overwintering rust pathogen must increase in numbers from where it survived the winter and move to soybean fields or other hosts. Soybean plants are susceptible during reproductive stages. The process of spore build up and movement may have to reoccur several times for rust to infect Northern regions. The viability of spores that travel long distances may be reduced by exposure to sunlight.

Environmental conditions: When spores land in a new field, local conditions must be optimum for rust to establish. Optimal conditions include: cloudy, rainy weather, temperature range of 60[degrees]F to 75[degrees] and a minimum of six hours of leaf wetness from dew or rain.

You can e-mail Pam Henderson at phenderson@farmjournal.com.