Rust Readiness Plan

Soybean rust experts recommend the following plan of action to be ready for rust this year:

1. Stay informed. Know how much rust inoculum survives the winter in the South. The most likely sources of rust spores for the Midwest are Mississippi, Louisiana, Texas and Mexico, due to weather patterns. The official source of current rust outlooks and updates is the USDA site, www.sbrusa.net. “This is where state rust specialists post their commentaries, so it's a great site to get a lot of information in a short time,” says Don Hershman, University of Kentucky plant pathologist. It also carries results from the sentinel plot system, a vital monitor of rust inoculum dispersion. Your state Extension office is the best source of local rust advice.

2. If there is a storm that could carry spores into your area, scout the lower portion of your plants' canopy with a 20X lens and know what you are looking for. “The trouble is, it's pretty tricky to spot at a low level, so inexperienced scouters may see rust and not know it,” says Greg Shaner, professor of botany and plant pathology and Extension specialist at Purdue University. “For that reason, sentinel plots are very important; they are scouted by knowledgeable people who get the word out in time.”

3. Maintain a solid relationship with your local fungicide dealer/applicator. “Keep up on the latest information on management of soybean rust with fungicides and watch the Web sites to see where soybean rust is developing,” advises Dean Malvick, assistant professor of plant pathology and Extension specialist, University of Minnesota.

“Remember that rust may not slowly creep toward you, providing ample warning,” Hershman says.

“Have your ear to the ground and have a plan, don't wait until rust is in your field,” says Carrie Lapaire Harmon, University of Florida Extension plant pathologist. “You don't need to purchase new equipment or buy fungicides right now, but do have a plan. That applies to anything in life.”

A New Theory

Last year, Asian soybean rust spores moved a great distance, but the disease did not. The fungus itself may have something to do with it. Rust spores were detected many times in each state of the northern soybean-growing region. Why then, was the disease not found there?

A University of Illinois study found that rust spores cannot survive more than two consecutive days of sunlight. The theory is that if spores were exposed to more than two days of a cloudless sky, they would be too weakened to cause disease, reports X.B. Yang, Iowa State University professor of plant pathology. A preliminary study suggests that rust spores may be sensitive to light, a factor previously unnoticed. Shaded rust spores resulted in a 95% rust infection, compared to only a 25% infection rate in unshaded spores.

This might explain why in South America the disease has always been found in lower portions of the canopy, despite the upper leaves being more vulnerable due to more dew and greater spore deposition.

Perhaps the cloud cover of rainy weather is what creates the connection between rust outbreaks and rainy weather. “If future research confirms these findings, the Midwest's past two summers' dry weather may have prevented the broad dispersal of spores from turning into a dangerous outbreak.

Fungicide Update

Last year, growers had six fungicides registered for use against rust, and another 11 had provisional Section 18 approvals in selected states.

This year's final list of approved fungicides are available on the EPA Web site later this spring: www.epa.gov/oppfead1/cb/csb_page/updates/soybean_rust.htm. Additional state Section 18 labels are available on state departments of agriculture Web sites.

One new rust fungicide in 2007 is Topguard, from Cheminova. With a triazole called flutriafol as its active ingredient, “it showed long residual activity in our University of Florida trials,” says Jim Marois, University of Florida professor of plant pathology. Topguard has Section 18 approvals in South Dakota and Minnesota.