Source: Kansas State University
For the first time ever, Asian soybean rust has been confirmed in a Kansas soybean field.
Kansas State University (K-State) researchers and the Kansas Department of Agriculture (KDA) have confirmed that a leaf sample from a soybean plant collected from a sentinel plot in Montgomery County has the disease.
"Asian soybean rust’s arrival in Kansas was expected," said Kansas Secretary of Agriculture Adrian Polansky. "And, we must continue to be vigilant, scouting for this disease in the coming years to ensure we detect it early enough for growers to take action to protect their crops."
The site where the positive sample was found is one of 20 that the U.S. Department of Agriculture (USDA), working with KDA and K-State, planted around Kansas to monitor for the disease.
For this year, this will only be a problem in late-planted soybeans," says K-State Research and Extension plant pathologist Doug Jardine. "There are 300,000-400,000 acres of late-planted soybeans this year that are potentially in danger. This represents about 10-15% of the state’s crop."
In university research trials during the 2005 and 2006 growing seasons, measured yield losses to soybean rust ranged from 6 to 32%, says Jardine. He anticipates that any potential yield loss in Kansas this year in the late-planted bean crop would likely be at the lower end of this range.
Soybean rust was first found in the U.S. Nov. 6, 2004, in Louisiana. Since then, it has spread to several states.
The disease is caused by either of two fungal species – Phakopsora pachyrhizi, also know as the Asian species, and Phakopsora meibomiae, the New World species. The Asian species is the more aggressive of the two and causes more damage to soybean plants, says Jardine.
The disease is spread by wind-borne spores capable of being transported over long distances. Asian soybean rust was first observed in Japan in 1902, and was found throughout most Asian countries and in Australia by 1934.
Any soybean field that has reached the R6 stage of development (full pod fill) is no longer in danger of the disease, Jardine says. 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 spreadsheet to assist producers in making the decision whether to spray. It is available on the AgManager Web page as "Economics of spraying soybeans."
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 because 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 KDA Web site.
"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," says Jardine. Scouting currently is more difficult in many areas of the state, due to the presence of two other similar-looking diseases: brown spot and bacterial blight, Jardine acknowledged.
For photos and further information on the disease’s symptoms, Jardine encourages growers to view a University of Missouri Web site.
Producers or scouts can submit samples of suspect soybean leaves for evaluation at K-State through any county or district Cooperative Extension Service office, 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. 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 also be found on the Plant Management Network Soybean Rust Information Center.