Winning the rust race

Scientists investigating the new race of dry bean rust found last fall in eastern North Dakota dry bean crops have gotten an early break in the hunt for disease resistance. Part of the credit for the early detection goes to plant pathologists at North Dakota State University in Fargo, according to Juan Osorno, dry edible bean breeder at NDSU.

"That was a big home run from Sam Markell and his group that they were able to find it and track it," he says. "They were on top of those things before it could take us by surprise."

No-show in 2009

The new race of bean rust, tentatively dubbed 20-3, was discovered by growers in Traill County, N.D., last year. Markell and fellow plant pathologist Rubella Goswami examined samples of the disease and determined that it was a new race and apparently able to attack every variety of dry edible beans grown in the Northern Plains.

The call went out for growers to watch for the new rust, with instructions on how to scout for it. More reports began to come in, all from Traill County.

At the time, Markell was concerned that, without any apparent resistance in area bean varieties, the disease could take hold during the 2009 growing season and spread to epidemic proportions. But that did not happen. Even the cool, wet spring, considered ideal weather for the spread of rust, did not lead to a severe outbreak.

"Rust is a big deal when it shows up early, and it definitely did not show up early," Markell says. "But it showed up pretty late, and it wasn't going to cause any damage."

This year's good fortune does not mean that 20-3 won't assert itself next year. Markell says it has proven it can overwinter, and therefore, it is "just as likely or unlikely again, next year, that we could have an epidemic."

Cure for the race

Bean varieties in the Upper Midwest have relied for many years on the venerable Ur-3 gene for rust resistance. It had served several times its expected lifespan of resistance to new varieties of rust until just last year, when 20-3 proved it had defeated the gene.

While producers were scouting their fields for early signs of the new rust, the NDSU research team was looking for a new resistance gene that could be bred into local varieties to replace Ur-3.

Osorno began gathering bean varieties from other regions of the United States, looking for varieties that might have a resistant gene not found in local varieties.

"When we sat down to design the experiment to test them, the conversation had come up before that Stampede had actually had another resistance gene in it," Markell says.

NDSU breeder Ken Grafton had made the first crosses for what eventually would become Stampede, Markell says. He had been trying to incorporate as many resistance genes as possible, and when it first was released, Stampede had three different genes. Though rust was controlled at the time, the breeders knew it eventually would break down.

"Just for the heck of it, we put some Stampede in there, just to test it again," Markell says. "We were pretty sure that the gene we were looking for wasn't in there anymore, but what did we have to lose? It's just a few more plants to plant."

They inoculated all their test varieties with the new race of rust. They wanted to find plants that resisted the damaging effects of the new rust.

"If they do, then Juan can start making crosses in the greenhouse, trying to integrate that resistance gene into our North Dakota varieties," Markell says.

This is square-one research. Getting from here to release can take several years, mostly because the breeder has to identify a resistant gene and then perform multiple crosses to get it firmly established into a plant without added "genetic baggage" and then prove that it is being passed on to the plants' progeny. It can be a long, painstaking process.

But the NDSU team caught a break.

"Low and behold, what we found is that some of the Stampede plants were resistant," Markell says.

Fast track

If the research team could establish a gene resistant to 20-3 in Stampede, it could take several years off of the process of getting a resistant commercial variety into growers' hands. It could mean the difference between planting resistant beans as early as 2012 or as late as 2018.

Looking back, Osorno says it should not have been a surprise that some resistance showed up in Stampede. He is intimately familiar with the breeding Grafton had done to create the variety.

"One of the last parents had three (resistance) genes," he says. "It looks like just one of them made it through the selection process and the other two got thrown to the side, probably just because we weren't looking for those."

But some of the Stampede plants were not resistant, telling Markell and the researchers that whatever gene was battling 20-3 was not being carried in every seed.

"The gene in Stampede wasn't fixed," he says. "That's not uncommon."

But it was enough to prompt a change in strategy.

"We planted a tremendous amount of Stampede -- hundreds of plants -- and we screened them with this new race," Markell says.

They culled the susceptible plants, about half of the population, and then conducted a progeny test.

"We took those plants that were resistant, we harvested the seed from them and then we planted four seeds of each plant. So now we've got, 500, 600 or 800 plants," he says.

If any plant's four seeds all would produce resistant plants, then that line could be considered uniformly resistant and having a fixed trait that resisted 20-3.

That growth cycle was completed in late September. All but about 80 lines were uniformly resistant. The resistant gene tentatively has been identified as Ur-11.

"So we've got, at least what we believe now, is really a pretty uniform seed that is resistant to this rust," Markell says.

And it already is a Stampede seed, with all the commercial traits it had before. There would be no need to engineer an entirely new multi-trait commercial variety to hold off the new rust. NDSU already had one in the greenhouse.

Despite the progress so far, Osorno warns that there are no guarantees. There are three critical propagation phases that must come off without a hitch to make enough seed available by summer 2012.