Late Summer Outbreak Leads To Increased Alert for Soybean Rust

From the University of Illinois...

Source: Glen Hartman (217)244-3258
Contact: Rob Wynstra (217)333-9446

URBANA--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, U.S. Department of Agriculture 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 said. "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 said. "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 said. "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.

"It is important for growers to keep an eye on what is going in Louisiana, Alabama, and Mississippi during the spring and early summer," Hartman said. "If we see lots of rust building up in those areas of the country during that part of the season, the threat from the disease reaching soybean fields in much of the Midwest will go up considerably."

Hartman notes that growers in Illinois have access to a great deal information on rust from groups such as the USDA, the Illinois Soybean Association, the North Central Soybean Research Program, the Illinois Department of Agriculture, and University of Illinois Extension.

"One of the most important tools for monitoring the situation is the soybean sentinel network," Hartman said. "This system for keeping track of the spread of rust is operating in more than 30 soybean-producing states. Detailed information is provided by the 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 http://www.sbrusa.net/. Reports from the sentinel plots in Illinois and other useful information on rust can be found at www.soybeanrust.org.

"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.

"After about mid-August the level of concern is much less," Hartman said. "The worst case scenario would be if rust shows up during mid-season and spreads all over the state. The arrival of the disease when the plants are flowering in June or early July would be a major concern. That means that growers would probably have to spray twice with a fungicide to control the problem."

Even so, the application of a fungicide has proven to be quite effective in controlling the disease. Additional research is under way to improve the timing of the applications. Other work is moving ahead on the long-term goal of developing soybean varieties with resistance to rust.

"There certainly is no reason for growers to overreact about the current situation," Hartman said. "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."