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Corn+Soybean Digest

Asian Rust Hits Brazil's Beans AGAIN

Thirty days had not passed since much of Brazil's soybean area was planted, and already Asian rust was making appearances.

The costly fungus showed itself in Brazil in 2002. Researchers say rust can yellow plants and cut their photosynthesis at any stage after plant emergence.

Embrapa researcher Cláudia Vieira Godoy estimates rust can cost producers more than 2 bu./acre in costs for controlling the fungus.

The costs for not controlling rust could be far greater, depending not so much on the spread of the wind-borne rust fungus itself, but on weather conditions that can make it act more aggressively.

Temperature, altitude and humidity all contribute. As a result, it's estimated that soybean farmers suffered a 10% loss nationwide last season, with up to 80% yield loss in the hardest-hit areas in the state of Mato Grosso. Participants at a meeting of farmers and researchers in the soy expansion state of Bahia estimated that state's losses at more than $100 million.

Producers in the northern state of Maranhão have typically gone from one fungicide application to three. As they do, fears of resistant varieties are mounting. That may be why the introduction of the “stainless steel” variety, developed from resistant African soy varieties, didn't work out this year.

Researcher Tiago Vieira Camargo of the Mato Grosso Foundation, a research entity of seed companies in that Brazilian state, says a new strain of the rust fungus has overcome the “stainless steel” variety's resistance, setting back its introduction.

Meanwhile, fungicide applications can control rust and greatly reduce its impact. At the moment, 11 chemicals are registered with Brazil's Ministry of Agriculture for rust control. But increased fungicide applications can be expensive. That's why one of the main efforts to overcome the threat has been to develop resistant or tolerant varieties. In addition, researchers are working to nail down the precise climatic conditions that encourage the spread of rust in order to put together a map of critical areas.

Rust is carried by the wind. It's apparently a long-haul flier, sailing over the Atlantic Ocean westward to Brazil from Africa. And it's not a picky assailant. Researchers have identified more than 90 known plant hosts. However, weather factors can greatly affect manifestation of the fungus. The ideal temperature for rust, say researchers, is 73Þ F., with a minimum of six hours per day in which there is water — dew or rain — on plant leaves.

Rust first shows up as dark spots on leaves, which then yellow and fall off. The symptoms are similar to those of several diseases. As a result, there have been numerous rumors of rust problems this year, which researchers are monitoring before diagnosing.

“With other diseases the farmer has been accustomed to 10-30% yield losses. Rust can cause (much greater) losses and we still don't have resistant varieties to lessen the problem,” says Godoy, from Brazil's National Soybean Research Center.

Godoy adds that rust has been detected in 90% of Brazil's planted area. “That's not to say there has been damage in 90% of the areas, but rather that it has been detected.”

Rust adapts fast, and Godoy worries about resistance developing in the three to four years it may take to develop a rust-tolerant or resistant variety. That's just what is allegedly happening to the vaunted “stainless steel” variety.

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