Soybean Rust in Kansas: To Treat or Not to Treat?

Soybeans at R6 or later are not in danger.

Published on: Sep 21, 2007

The news that Asian Soybean Rust has been found in a sentinel plot in Montgomery County begs the question: how do I know if I need to spray a fungicide for the disease?

Any soybean field that has reached the R6 stage of development (full pod fill) is no longer in danger from the disease, says Doug Jardine, plant pathologist for K-State Research and Extension. Decisions to apply a fungicide on fields that have not reached that growth stage need to be made on a field-to-field basis.

K-State Research and Extension has developed a calculator spread sheet to assist producers in making the decision to spray or not. It is available on the AgManager.info Web page www.agmanager.info as "Economics of spraying soybeans" or directly at: http://www.agmanager.info/crops/prodecon/production/decision/default.asp.

The decision to spray is dependent on a combination of growth stage, application costs, expected selling price and the yield that will be saved by spraying. Given the level of disease currently being found, yield savings would likely be no more than about 10%.

If a fungicide application is deemed necessary, producers are encouraged to use a triazole fungicide since the mode of action will have some curative effects on infections already in progress. Triazole fungicides currently labeled for use on soybean rust in Kansas include Alto, Caramba, Domark, Folicur, Laredo, Punch, Tilt and Topguard.

Growers can access information about fungicides currently registered for use in Kansas from the Kansas Department of Agriculture Web site at www.ksda.gov/pesticides_fertilizer/content/288.

"To scout for soybean rust, you should arbitrarily collect or observe a minimum of 100 leaflets from the lower canopy (older, main-stem terminal leaflets) in each field," Jardine says. "Areas of the field that may be shaded, especially from the morning sun, are good places to look for rust. To observe pustules, a minimum 20X hand lens is needed, but 30X is better. Pustules will be found on the bottom side of the leaflets and have the appearance of small volcanoes within the lesion."

Scouting is currently more difficult in many areas of the state due to the presence of two other similar looking diseases, brown spot and bacterial blight, Jardine acknowledges.

For further information on the disease's symptoms and photos, Jardine encourages growers to view a University of Missouri Web site: http://extension.missouri.edu/explorepdf/agguides/crops/g04442.pdf .

Producers or scouts can submit samples of suspect soybean leaves for evaluation through any county or district cooperative Extension service offices, Jardine says

He recommends:
• Send 10-15 suspect leaves.
• Place the leaves in a zipper-style plastic bag with a tight seal, and put that bag in a second such bag. Seal both bags securely.
• Attach a note or write on the outer bag that this is a soybean rust sample.
• Do not add wet paper towels or other types of moisture to the bag.
• Try to mail it in the Monday-Wednesday time frame to avoid the possibility of the samples spending the weekend in a hot post office loading area.

Soybean growers and industry people can track the spread of soybean rust on the USDA Legume PIPE Web site at www.sbrusa.net . This site also contains weekly commentaries from Extension specialists in each state on crop growth and development, disease progress, scouting and fungicide application recommendations.

Additional information on soybean rust can be found on the Plant Management Network's Soybean Rust Information Center at http://www.plantmanagementnetwork.org/infocenter/topic/soybeanrust/ .