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Mississippi Agricultural News

Zimbabwe expert shares
soybean rust knowledge

By Bonnie Coblentz

MISSISSIPPI STATE -- Asian soybean rust came to Zimbabwe in 1998, and researcher Clive Levy was on the front line battling the disease in commercial fields in that African nation.

Levy was in Mississippi in mid-June, searching for the disease in the state's fields with Mississippi State University Extension Service personnel. As of June 30, rust has not been found in Mississippi this year.

"When Dr. Levy got here, he told me that rust was in Mississippi and we would find it," said Extension soybean specialist Alan Blaine. "We kept him in the field for six days and traveled hundreds of miles across the state, but he did not find it.

"We pulled four samples in the south Delta, and he was positive that some of them were rust, but when we took them to the lab, all four were negative," Blaine said. "He's a world-renowned expert who thought we had found rust in Mississippi, so I think this shows how difficult it is to identify."

The Soybean Promotion Board sponsored Levy's trip to Mississippi and is helping lead the fight against soybean rust. Mississippi is monitoring sentinel plots twice a week to determine if rust has arrived. When it does, appropriate recommendations will be made to growers.

Levy, a researcher with the Commercial Farmers Union of Zimbabwe, gave a presentation during his visit to MSU researchers and Extension agents, as well as growers, to teach them how to identify the fungus. He also talked about treatment options and his experience with the disease.

"At its early stages, it can be confused with bacterial blight, bacterial pustule, brown spot or mustard spot, or insect damage such as from spider mites," Levy said.

Early symptoms include yellow discolorations of the upper surface of the lower leaves, small gray- to russet-colored spots and pustules that rapidly spread from the lower leaves to the upper leaves. More advanced stages display yellow leaves, brown scarring on the undersides of leaves and a diseased appearance of the plant.

"In hot, dry weather, a cloud of spores forms in extreme cases within and above the canopy of a diseased field," Levy said. "If you're standing in it, the spores can burn your eyes."

He said the fungus rapidly defoliates diseased plants, and untreated, completely destroys the crop.

"While it is a devastating disease, chemical control can be very successful," Levy said.

He said Zimbabwe planned a national strategy when rust arrived, and the country was able to fight it effectively. Keys to the fight were monitoring for the disease using trap or sentinel crops and the use of fungicides. He said researchers in his country have not been able to control soybean rust with agronomic practices, such as varying row spacing. They are now focusing efforts on screening for the disease and breeding resistance into soybean varieties.

"We brought Dr. Levy in to allow us to learn from their experiences in Zimbabwe," Blaine said.

He said the United States has several good chemical options available to combat rust, but some labels were late getting approved, causing concern about available supplies. Historically, soybean producers have been unwilling to spend a lot of money spraying for soybean diseases, Blaine added, but the appearance of rust will make it a necessity.

"I think as we use fungicides in this crop more judiciously than in the past, we will be controlling not only rust but other diseases and we will see our yields increase," Blaine said.

The timing of fungicide applications is very important. Blaine said the chemicals have no benefit if sprayed before the disease arrives, and it appears that the most beneficial treatments begin when plants start setting small pods.

"No one should be afraid of rust," Blaine said. "We've got good chemical options and some day down the road, we'll have varietal resistance. The problem, if any, will come from not spraying at all or not spraying at the proper times."

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Released: June 30, 2005
Contact: Dr. Alan Blaine, (662) 325-2701

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