June 18, 2009

Asian soybean rust jumped the gun this season, with the fungus showing up on soy in Alabama and Louisiana earlier than ever before.

 

Asian soybean rust, which is caused by a fungus, can cause near-total yield losses through premature defoliation of infected plants. Application of chemical fungicide is the only known method of controlling rust.

 

Soybean rust was discovered on soy in Iberia Parish, La., and Baldwin County, Alabama, between June 5-8, following a period of extremely wet weather that was very favourable for development of the rain-loving plant pathogen.

 

"Both reports are several weeks ahead of previous first finds on soyn in each state," said Iowa State University plant pathologist Daren Mueller. "Second, unlike the past few years, there has been ample rain for soybean rust to increase inoculum [spores]."

 

Some parts of the Deep South received more than a foot of rain during May, which provided ideal conditions for Asian soy rust development. Rust has also been found on the prolific weed kudzu in Florida and southern Georgia thus far in 2009.

 

"These finds are cause for concern, but not alarm," said University of Kentucky extension plant pathologist Don Hershman.

 

However, a turn toward hot, dry weather in June has since slowed the spread of Asian soy rust.

 

"We are in a severe drought in most of the state," said Louisiana State University professor Clayton Hollier on Monday. "Soy is fine for now, but will need rain very soon."

 

Arid conditions quickly kill soybean rust spores, which act as seeds for the disease and are spread by the wind.

 

Although the US Department of Agriculture's official rust infection forecast says strong winds could lift rust spores from infected areas of Florida and Georgia, deposition of those airborne spores by thunderstorm rains are "unlikely."

 

According to a soybean rust spore movement predictive model maintained by Iowa State University and St. Louis University, spore dispersal has been limited to the far southern US so far this spring.

 

"For now, even with the predicted increased spore movement, the chances of spores getting to Iowa in the next month is very low, because there are no strong sources of inoculum in Arkansas, Oklahoma, Texas or Mexico," concluded Mueller.