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Corn+Soybean Digest

Late Summer Outbreak Leads To Increased Alert For Soybean Rust

Although soybean rust has caused little or no damage during the past two years, growers are advised to be on high alert during the coming season. One particular concern is a major change in the 2006 distribution of soybean rust and the over-wintering areas for the fungus that causes the disease, according to Glen Hartman, USDA plant pathologist in the Department of Crop Sciences at the University of Illinois.

"The current situation is different than it has been during the last two years," Hartman says. "There has been a marked increase in the size and location of the areas where the fungus occurs since moving into the upper Midwest late in the 2006 growing season and where it will potentially overwinter this year. That presents the real possibility that there could be an increased threat to the major growing areas during 2007."

In 2005, the fungus that causes the disease overwintered only in a few areas of southern Florida. Last year, the fungus stayed mostly in parts of Florida, Georgia and Alabama before spreading into the lower Mississippi River Valley and to the Midwest late in the growing season.

"During the 2006 season, rust infection and spores occurred over large sections of Louisiana," Hartman says. "This is especially significant because spores from that part of the country have a direct pathway up the Mississippi River Valley into Illinois and other major soybean growing areas in the Midwest. The situation has changed enough that soybean growers will need to be on heightened alert during the early part of the growing season."

Hartman notes, however, that there still could be a hard freeze that would serve to greatly reduce the risk of a major outbreak.

“But, if we have a mild or even average winter, the situation could be set up for an earlier development of rust,” Hartman says. “At the same time, rust could still remain in check if we have a very dry spring. The outcome will be dictated by the early season weather in the south. Right now, rust on kudzu has been found much farther north and west compared to the previous two winters.”

He points out that any spread of the fungus into the Midwest soybean-growing region will most likely be preceded by a major build up of rust in the South. The result would be a huge amount of spores that could be swept north with the prevailing moisture from the Gulf of Mexico.

"One of the most important tools for monitoring the situation is the soybean sentinel network," Hartman says. "This system for keeping track of the spread of rust is operating in more than 30 soybean-producing states. Detailed information is provided by USDA on its soybean rust information website. A map on the website is used to clearly indicate each county where rust has been diagnosed."

The map covering the entire country is located on the USDA's website at Reports from the sentinel plots in Illinois and other useful information on rust can be found at

"Growers should check the map frequently as the spring season moves along," Hartman said. "Rust will not show up in Illinois out of nowhere. The key will be when the map begins to light up in northern Kentucky and Arkansas and southern Missouri. When that happens, they can begin to take appropriate actions based on their own risk tolerance."

Another important consideration will be the timing of an impending outbreak of rust. Hartman notes that the disease could cause some significant problems as late as the first part of August.

"There certainly is no reason for growers to overreact about the current situation," Hartman says. "The key is for them to maintain their vigilance. They should take the time to closely monitor the situation through the first part of the growing season. If a problem develops, they will have plenty of time to take whatever steps fall within their own risk tolerance."

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