June 24, 2009

Delayed plantings and heavy spring rains have put much of the US soy crop at an unusually high risk of damage from Asian soy rust, a yield-robbing fungus that has already begun to infect soy weeks earlier than ever before.

 

"These early finds mean that soy rust could be a real threat to the soy crop, if the disease continues to progress," said Iowa State University plant pathologist Daren Mueller.

 

Thus far in 2009, soy rust has been found in 23 counties of Alabama, Florida, Georgia, Louisiana and Texas. It infected a record 392 counties across 16 US states in 2008, but caused little yield damage because the disease developed late in the growing season.

 

"This can be a devastating plant disease. Yield losses of 40-50 percent are possible in soy when the disease invades a field early...and no fungicides are used to prevent spread of the pathogen," warned Alabama plant pathologist Ed Sikora, after the disease was found in an Alabama soy plot June 8. The Alabama discovery occurred seven weeks earlier than in 2008.

 

The Asian soybean rust fungus (Phakopsora pachyrhizae) thrives in cool, moist, overcast weather and is considered one of the world's worst plant diseases. The fungus was first found in the continental US 4 1/2 years ago, but is killed by frost and generally does not overwinter north of the Gulf Coast.

 

This year, in addition to developing sooner, the disease is also targeting a soy crop that was planted later than normal, providing the pathogen an even greater window of opportunity. The US Department of Agriculture said Monday only 91 percent of all intended soy acreage had actually been seeded as of June 21, compared to the seasonal average of 95 percent.

 

"There are still nearly 7 million acres to plant, with 65 percent, or 4.5 million, scattered through Illinois, Indiana, Kentucky, Missouri and Tennessee," said Benson Quinn Commodities analyst Jon Michalscheck.

 

A total of 11 million acres - or nearly 14 percent of the nation's total crop - will have been seeded after June 15, when soy is normally considered "late" by most agronomists.

 

"Late planting in soy really doesn't affect the disease situation much," said University of Kentucky extension plant pathologist Don Hershman. "Usually, the only disease where there could be an increased risk is with soy rust."

 

Soy is most susceptible to rust infection after the plants have flowered and before pod-set, which normally occurs during July and August.

 

Rapid applications of chemical fungicide are the only known method of controlling rust epidemics, and Sikora is already recommending that some Alabama farmers spray their fields immediately.

 

"A storm pushing out of the Gulf [of Mexico] could pick up rust spores from along the Gulf Coast and spread those spores any number of miles," said Sikora, who added that reports from other countries suggest the disease can spread 100 miles a day.

 

A rust infection forecast released Monday by the USDA said, "Light winds will isolate spore transport to the Gulf Coast with the main exception of eastern Louisiana. Spore transport is also possible in eastern Texas."

 

Infected kudzu found north of Houston over the weekend is particularly problematic for the US soy industry, as Texas - due to prevailing wind circulation patterns - has long been considered a key jumping-off point for any potential spread of rust from the South to intensive soy growing areas of the Midwest, as was the case in 2007, when the disease first reached Iowa.

 

"According to the observed disease occurrence in the Great Plains in 2007, it is clear that the western pathway of soy rust became a major pathway in soybean rust's northward dispersal," said ISU extension plant pathologist X.B. Yang. "Spore sources in Texas and Mexico likely may have greater chances to reach [the] North Central region compared with spore sources in the southeastern states."