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Corn+Soybean Digest

Fungicide only option for rustproofing soybeans

Soybean farmers routinely treat their fields with herbicides and insecticides to ward off crop-damaging weeds and bugs. Before long, growers might need to add a third chemical compound to their spraying ritual, says Greg Shaner, a Purdue University Extension plant pathologist.

Fungicides could play a larger role in future soybean production, especially if a devastating fungal disease enters the continental United States, Shaner says. "Farmers who've mainly grown corn, soybeans and, possibly, wheat, probably have not had that much experience using fungicides," he says. "Virtually all of them use herbicides and they may use insecticides but not fungicides, other than for seed treatments."

That could change with the possible arrival of Phakopsora pachyrhizi, also known as soybean rust. The disease attacks a soybean plant's foliage, causing leaves to drop early and disrupting pod setting. Crop losses as high as 80 percent are possible in untreated soybean fields.

For decades soybean rust was confined to Asia. The disease appeared in Africa in the late 1990s and then crossed the Atlantic Ocean into South America in 2001. Since then, researchers have seen soybean rust march north through Brazil and leap the Amazon River.

Plant pathologists and agronomists believe soybean rust eventually will reach the United States, although they aren't sure how or when. The disease is spread when rust spores travel from one soybean field to another. Spores can be carried long distances by wind, as well as on clothing. Government officials are concerned terrorists could release the pathogen in the U.S. as a biological weapon.

To date, fungicides are the only effective means for controlling soybean rust, Shaner says. Most Midwest farmers don't use fungicides because they either aren't growing specialty crops or they don't see the economic benefit of adding fungicide treatments, he said.

Applying fungicides to a soybean crop adds between $16 and $25 per acre to production costs, Shaner says.

"Fungicides have been used for a long time, but mainly on higher value crops such as fruits, vegetables and turf," he says. "In the economics of field crop production, fungicide applications generally have not been justifiable. But with the likelihood that soybean rust will arrive in the U.S., fungicides are going to be essential for control of the disease.

"The other way of controlling the disease -- and the more desirable way -- is rust-resistant soybean varieties. Unfortunately, we don't have any. Until such time as we have resistant varieties, farmers are going to have to use fungicides." Most fungicides are sold as liquids. The chemicals are diluted with water or oil and then sprayed over plant foliage.

Fungicides fall into two classes: protectants and systemics. Protectant fungicides coat plant surfaces and block fungi from entering plant tissue. Systemic fungicides are absorbed by the plant and provide internal protection against fungal diseases.

"One thing about fungicides -- and it depends on the product you're using -- is that good penetration of the plant canopy is necessary for effective control," Shaner says. "With a lot of herbicides if you get just a few drops on the weed, that's sufficient. With fungicides you've got to get more complete coverage.

"If soybean rust comes early in the season and the plant is continuing to grow, you'll have the disease on the lower canopy. You'll want your fungicide to penetrate down into the canopy. For that reason, spray nozzle types, nozzle configurations, spray pressures and spray volumes might be a little different than what soybean producers are accustomed to using for herbicide spraying."

Timeliness is important when applying fungicides, Shaner says. "You've got to spray early when soybean rust is just starting to develop," he said. "If you wait until the problem is obvious, it's too late. Treatments need to be more preventative than curative."

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