Source: FarmWeek, Illinois Farm Bureau via Checkbiotech

In an attempt to control a “clever” adversary, Brazilian researchers are working to build a genetic arsenal that will allow farmers to continue a long-term fight against Asian soybean rust. Development of disease-resistant cultivars has become a primary goal in Brazil, according to Jose Francisco Ferraz de Toledo, a scientist with Brazil’s Embrapa soy research center.

But rust presents a special challenge because of its high degree of variability — it can develop several races within a short period — and high spore “dispersal capacity.”

To date, Embrapa has identified several highly tolerant soy breeding lines. But because of rust’s variability, researchers also are studying genes that provide lesser levels of tolerance but could bolster “straight” resistance traits.

One variety now in final yield trials has shown continued resistance and should be available in another year for use in central Brazil. But Toledo noted several initially promising resistant genes now are susceptible to rust.

“The rust seems to be clever, so we need to have a fungicide at hand,” Embrapa head of rust research Jose Tadashi Yorinori advised U.S. scientists and producers visiting the Embrapa facility last week. Because of the diversity of Brazilian varieties, Toledo reported, “we cross just about everything with everything in the program,” in search of materials with added tolerance or that may slow or delay the disease cycle, thus allowing farmers to “escape with just one (fungicide) spraying.”

They are aided by what Yorinori termed an important “clue” to resistance. The disease does not kill leaf tissue around its tell-tale, spore-bearing pustules, although tan-colored, generally harmless lesions appear around the pustules.

Embrapa scientists discovered that in some lines exposed to rust, brown or red lesions appear as leaf tissue dies, indicating resistance.

“The rust fungus needs living tissue to grow,” Purdue crop scientist Greg Shaner explained. “By sacrificing a few cells, the host plant is protecting itself. You get a lesion, you get damage, but you don’t get very much sporulation (spore release), so the epidemic isn’t able to build up.” Though Embrapa focuses on varieties suited to Brazil’s specific climatic and agronomic conditions, Pioneer breeder Paul Stephens noted “the genes will transfer,” opening the door to biotech rust solutions.

Given uncertainty about prolonged natural resistance, rust tolerance would appear to be the “ideal trait” for a biotech approach, he suggested.

Incorporation of genetic material from other species also could offer promise “if (soybean resistance) traits don’t hold out,” Stephens said. One major question is whether the economic impact of a soybean disease epidemic would outweigh U.S. consumer concerns about GMO crops.

In that regard, Brazil, which has grappled nationally with biotech approval even as southern Brazil growers routinely raise Roundup Ready beans, could become an important proving ground for GMO rust resistance.

“The South Americans now have an issue. If they come up with a biotech solution, that could bode well for getting it approved and into domestic use,” Stephens told FarmWeek. – Martin Ross