WESLACO, Texas -- Asian soybean rust -- a potentially serious fungus of soybean crops -- has now been found in an area of Texas where prevailing winds could help spread the disease.

While scientists are not sure how the disease will behave in this part of the world, some are concerned that fungal spores from South Texas will ride the winds to the Midwest's multi-billion dollar soybean crop.

Marvin Miller, a Texas Agricultural Experiment Station plant pathologist in South Texas, found the fungus on soybean leaves Feb. 14 in a research field plot at the Texas A&M University System Agricultural Research and Extension Center at Weslaco.

A preliminary diagnosis made at the Texas Plant Disease Diagnostic Laboratory in College Station was confirmed by a USDA laboratory in Beltsville, Md., Miller said.

"Soybean production here in South Texas isn't significant, but this is of great concern to the production areas of the Midwest," Miller said. "In addition to soybeans, cowpeas, green beans, and other edible legumes could be affected by this fungus which causes defoliation and yield reduction."

The soybean rust fungus (Phakopsora pachyrhizi) was first introduced into the U.S. in Florida in 2004, likely aboard hurricane winds, but confined itself to the Southeast, according to Tom Isakeit, a plant pathologist with Texas Cooperative Extension in College Station.

South Texas rust find likely a new introduction

"This finding in South Texas is probably a new introduction," Isakeit said. "And what it suggests is that soybean rust could take the so-called 'rust corridor' to points possibly as far north as Canada."

Prevailing winds have been blamed for taking a number of other rust diseases on a northerly path, including southern rust of corn, stem rust of wheat, and common rust of corn, he said.

Joseph Krausz, a scientist at Texas A&M University's department of plant pathology, said confirmation of soybean rust in both Mexico and Texas is very significant.

"The recent occurrence of soybean rust in South Texas and in the Mexican states of Tamaulipas and San Luis Potosi late last year may very well be the first step in the pathogen becoming established, or endemic, in the 'rust corridor,' which historically has been the pathway of seasonal dissemination of a number of rust diseases from south to north," Krausz said.

Environmental conditions will play a major role in influencing the further establishment of the pathogen in the corridor, he said.

The soybean rust found in Texas on Valentine's Day was in a one-acre research field plot in Weslaco that was part of an irrigation study. Isakeit had checked the field in December and found no symptoms of the fungus.

"Infection must have happened after my visit in December, unless plants were already infected but were not yet showing symptoms," Isakeit said.

Trying to pinpoint the source of the spores can be difficult, he said.

"Rust spores can be carried by wind currents hundreds or even thousands of miles, even across oceans," Isakeit said. "They are eventually brought down to earth by rain, and a leaf that's been wet for several hours is needed in order for a spore to germinate and infect a plant."

But once a plant is infected, the fungus can multiply rapidly, Miller said.

"That's why growers need to be vigilant. There are fungicides that are registered for use on soybean rust, but early detection and treatment are key," he said.

Resistant varieties of soybeans are being developed, but will not be available to growers for several years, according to the American Soybean Association's Web site.

Monitoring in Weslaco to continue

The field in Weslaco where the fungus was found has been harvested and plowed under, but Miller said a "sentinel" plot of soybeans will be planted to serve as an early-warning system for growers.

"A few rows of soybeans will be planted shortly which we'll be checking throughout the spring," he said.

Isakeit said other bean crops grown in the area, including commercial green bean and cowpea fields, also will be monitored.

"Surveying these fields will provide information to get the big picture of how this fungus behaves in this part of the world," he said. "We know it's caused major losses in Asia and other parts of the world, but we don't really know how it will affect crops here. The weather and prevailing winds this spring will play a big role in what happens, but if South Texas bean crops have a rough year with soybean rust, there will be the potential for a bad year further north."

The U.S. is the world's largest producer and exporter of soybeans, with a farm value of $18 billion in 2003-04, according to USDA.

Soybeans are used in 90 percent of the total U.S. oil seed production and are grown throughout the Midwest, with the heaviest concentration of the crop in Illinois, Indiana and Iowa.

SOURCE: By Rod Santa Ana III of Texas A&M University AgNews.