Source: AgAnswers

The report of soybean rust in Louisiana means Midwest farmers may have to deal with the problem next growing season if the right mix of plant hosts and weather develops, according to Purdue University experts.

"We don't yet know how widespread the infection is, but if you find it in one location, it's likely that it's also on other nearby spots that you've not yet detected," said Ray Martyn, a professor and head of the Department of Botany and Plant Pathology. "If it's not controlled in Louisiana, then it could be a problem for Indiana farmers next summer."

Martyn was part of a team of Purdue researchers that plotted the potential rate for soybean rust spread in the United States. The work was done under contract for the U.S. Department of Agriculture as part of a pathway analysis to help identify areas of susceptibility and where more research was needed.

Martyn said soybean rust could easily make the trek from the southern states to the Midwest during the next growing season if conditions are favorable.

The fungus, reported Wednesday (Nov. 10) for the first time in this country, could survive the winter if it finds one of 40 known hosts for the disease. Kudzu, a common nuisance plant in the South, is one of those possible hosts.

"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.

Even if soybean rust takes up permanent residence in this country, it also needs favorable growing conditions to spread. "It needs humidity and moderate temperatures," Martyn said.

Greg Shaner, a professor of botany and plant pathology who also worked on the pathway analysis, said 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."

There are no soybean varieties grown in the United States that are resistant to soybean rust, so using fungicides is the only control measure currently available to U.S. producers. That could pose a problem next year if the disease spread is prevalent and fungicide supplies are low.

"Right now fungicides are applied to between 43 and 44 million crop acres in this country," Martyn said. "The United States produces about 78 million acres of soybeans annually, which means that the demand for fungicides could greatly escalate."

There are currently two fungicides approved for soybeans in the United States, but Shaner has started a process to get officials to approve 10 more fungicides for use in Indiana.

Martyn and Shaner said even if soybean rust stays in the United States, 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."

Shaner said soybean rust is something that should be treated only when the problem appears.

"Yield loss from soybean rust is the most damaging if it occurs early on in the plant's development," he said. "It generally shows up at the beginning of flowering, but can occur anytime during the growing season."

More information on soybean rust is available at
http://www.agriculture.purdue.edu/soybeanrust or at
http://www.ppdl.purdue.edu/ppdl/soybean_rust.html .