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Purdue lab puts suspect soybean leaves to the test

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Written Friday, July 22, 2005  

The Purdue University Plant and Pest Diagnostic Laboratory is geared up to assist growers in identifying Asian soybean rust, a disease that could cause economic losses nationally as high as $2 billion annually.

The laboratory and the Indiana Soybean Board have declared early identification of soybean rust a top priority. The board is paying the $11 handling fee for all samples of suspected soybean rust sent to Purdue from farmers and others in agribusinesses within Indiana.

"Because of soybean rust's potential impact and the need for fungicide application as soon as the disease becomes established in a field, it's critical that the time between sample collection and identification be as short as possible," said Gail Ruhl, senior plant disease diagnostician and co-director of the laboratory.

"Farmers need to inspect their fields for symptoms of the disease, which first appear as small dark spots on leaves in the mid- to lower canopy of the plant," she said. "Volcano-shaped lesions, called pustules, develop in these spots on the leaf underside. The pustules produce spores that can cause more infection."

Greg Shaner, Purdue Extension plant pathologist, said early detection is imperative.

"Ideally, a fungicide should be applied just before rust arrives, but it's impossible to determine this event," Shaner said. "A fungicide can be applied after rust has infected a field if the level of rust is still very low."

Lab tests to determine whether a leaf sample is infected with soybean rust are necessary because in the early stages rust symptoms can be confused with a number of other diseases, Ruhl said. This means that farmers must carefully examine their plants for any of the characteristic pustules on the leaves.

If farmers find suspect signs of rust, they should collect at least 20 leaves, press them flat between paper towels or newspaper, place the leaves in a self-sealing plastic bag, and place that bag inside another self-sealing plastic bag.

Samples need to be carefully packaged to ensure their condition is good enough for correct identification when they arrive at the lab. Also, the packaging must be such that it won't allow accidental spread of the fungus, Phakopsora pachyrhizi, which causes Asian soybean rust.

"Don't wet the paper towels because extra moisture promotes mold growth that will compromise the sample and make identification difficult, if not impossible," Ruhl said.

The plastic bag must be labeled with the date, collector's name and phone number and attached to a completed Soybean Rust Submission Form, available at http://www.ppdl.purdue.edu/ppdl/pubs/soybean_rust_submission.pdf .

The bag and form must then be packaged in a box or padded envelope that has all the seams sealed tightly with tape. The second bag and the tight sealing are to prevent the escape of any spores.

The packaged sample should be shipped overnight or hand-delivered to the Purdue University Plant and Pest Diagnostic Laboratory, 915 W. State St., Room 101 LSPS, West Lafayette, Ind. 47907-2054. The lab is open 9 a.m. to noon and 1-5 p.m. Monday through Friday.

"If the person sending the sample will call or e-mail us to tell us it's on the way, we'll be watching for it and begin the identification testing process immediately," Ruhl said. "It's best not to send it on a Friday since that means the sample would be delayed in transit over the weekend before testing can begin."

Asian soybean rust was identified for the first time in the United States in November 2004 in Louisiana, according to the USDA.

Because experts believe that winds from Hurricane Ivan last fall transported the disease-spreading fungal spores to the United States from South America, scientists and farmers now anxiously wait to see whether the current storm season will disperse the pathogen northward from infected sites in the southern United States.

Tracking the movement of the disease is a task that scientists and farmers must handle jointly, Shaner said.

It takes approximately nine days between when the spores arrive and disease symptoms are readily recognizable.

After the initial confirmation in Louisiana last November, Asian soybean rust was confirmed in eight other states in 2004, including Florida, Georgia, Alabama, Arkansas, Mississippi, Missouri, South Carolina and Tennessee. The fungus survived the winter in Florida on a non-native plant called kudzu and then began spreading across the South.

"So far this year, there's no report of soybean rust anywhere other than Alabama, Florida, Georgia and Mississippi," Shaner said. "The path of Hurricane Dennis was perfect for bringing spores from confirmed sources up into the Midwest, and possibly into Indiana. But because there's no abundant source of disease in the South, we would anticipate it would be a very light spore load."

An Asian soybean rust invasion could raise production costs an average of $25 per acre, according to USDA Economic Research Service experts, and the yield losses could be as much as 10 percent nationally. Individual fields can suffer almost total yield loss if rust comes in early and is not controlled.

However, the yield could increase by as much as .9 percent in fields treated with rust fungicides because the treatments for that disease also may be effective against some other pathogens, which normally aren't destructive enough to spray for individually, Economic Research Service experts estimate.

This benefit is more likely to be realized in southern states than in the Midwest, Shaner said.

For more information about soybean rust, log onto the Purdue Plant and Pest Diagnostic Laboratory Soybean Rust Web Page, at http://www.ppdl.purdue.edu/ppdl/soybean_rust.html .

Growers can receive regular updates about the disease by calling the Purdue Soybean Rust Update phone line. Dial the toll-free Purdue Extension hotline at 1-888-398-4636 (EXT-INFO) and ask for the update. After hours and on weekends, callers should dial the number and then zero when they hear the recorded message. The update message -- and a similar message from Ohio State University Extension -- also is available on the Ag Answers Web site.


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