November 9, 2004
From:@griculture Online News, 11 Nov 2004 [edited]

Rust could overwinter in Florida, Gulf Coast states

As federal and Louisiana plant disease experts flood to the scene of the first confirmed case of Asian soybean rust in the United States, university experts offer insight on how quickly the fungus might spread to soybeans in
other areas of the US.

Soybean rust could easily move from the southern states to the Midwest during the next growing season if conditions are favorable, says Ray Martyn, a Purdue University plant pathologist who was part of a team of researchers that plotted the potential rate for spread of the disease in the US for the United States Department of Agriculture (USDA).

According to Martyn, if it is found in 1 location, it is likely that it is also on other nearby spots not yet detected. Moreover, if it is not controlled in Louisiana, then it could be a problem for Indiana farmers next summer.

A look at the long term normal temperatures for the area affected by rust in South America shows the disease could overwinter down to latitude 30 degrees south, according to a 2004 research report by S Pivonia and XB Yang, Iowa State University plant pathologists. In the US, the possible overwintering zone is up to about latitude 28-29 degrees north in Florida and in the western Gulf of Mexico coastal regions, the researchers said in the report.

To overwinter, the disease would need to infect one or more of 40 known hosts for the disease. Kudzu (_Pueraria lobata_), a common weed, is one of them. Others include yellow sweet clover (_Melilotus officinalis_), winter vetch (_Vicia villosa_) and blue lupine (_Lupinus angustifolius_). All are cultivated as winter forage and silage crops in the south.

"If a plant stays moderately green all winter and it's a host for the disease, then soybean rust could overwinter in the extreme southern parts of the United States," Martyn said. "In the event that _P. pachyrhizi_ [Asian soybean rust] becomes well established around the Caribbean/Gulf of Mexico basin, aerial transport of soybean rust spores into the continental interior of North America is likely to occur each spring." Even if soybean rust is established in this country, it also needs humidity and moderate temperatures in order to spread, Martyn adds. In addition, the disease prefers cooler weather. Greg Shaner, Purdue plant pathologist, says the spring and summer southerly breezes could easily transport soybean rust spores from state to state as the growing season progresses.

"Studies of soybean rust in China and Brazil can provide some insight into what we can expect, but our climate and growing conditions are not the same as in those countries," Shaner said. "For instance, China doesn't have the strong jet stream winds that travel our continent. Those winds can transport spores quite readily."

Martyn and Shaner say even if soybean rust stays in the US, the problem could vary greatly from year to year. "It is unlikely to overwinter in Indiana, so it would have to move up from the south each year," Martyn said. "If southern farmers have good control measures and a growing season turns out to be hot and dry, then you would not expect to have a widespread soybean rust problem that year."

[Surveys within the past few days have confirmed that _Phakopsora pachyrhizi_ [Pr], the fungus that causes soybean rust, was detected in research plots at Louisiana State University and on farms near Natchez, Mississippi. Research by S Pivonia and XB Yang, Iowa State University, Department of Plant Pathology, Ames, Iowa has shown that, globally, soybean production regions can be divided into 2 types: type 1 areas, where the disease can survive year-round on suitable hosts, and type 2 areas, where disease occurrence depends on an external source of inoculum after long distance dispersal from a source area. Most regions where SR is known to occur are in areas of type 1. After soybean rust enters the United States, the fungus is likely to overwinter in parts of Florida and southern Texas. Major soybean production regions in the United States and central Argentina are, like Central China, type 2 areas.

Link: <http://www.seedquest.com/News/releases/2004/april/8381.htm> - Mod.DH]