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California wheat growers challenged by stripe rust

California wheat growers challenged by stripe rust

California wheat crops looks good as harvest begins. However, stripe rust attacking more resistant varieties. Genetic breakthrough by UC wheat breeder offers hope of introducing stripe rust resistance into older, new varieties.      

California’s wheat harvest is underway.

Although there are widespread reports of high levels of stripe rust in several varieties and blank heads due to frost, the 2011 crop is expected to be a good one.

There are 750,000 acres of wheat in the state this year with a little more than 100,000 acres of durum/pasta wheat grown mostly in the Imperial Valley, which started its harvest in mid-May.

Growers are currently harvesting central San Joaquin Valley wheat fields for silage, and Steve Wright, farm advisor, University of California Cooperative Extension, Tulare and Kings counties, says growers are reporting “good yields.”

He expects the same information for fields harvested for grain when combines start rolling in the near future.

To justify his prediction, Wright repeated the ageless axiom that bad weather for cotton planting means good conditions for heavy grain yields. It has been a cool, wet spring, far from ideal for cotton planting, but near perfect for finishing fall-planted cereal grains.

“Varieties not impacted by stripe rust are looking real well,” he said, adding, however, that frost at the boot state in some fields is showing up as blank heads.

He predicts about 50 percent of the wheat acreage in his area will be harvested for silage and the other half for grain. It once was two-thirds silage and a third for grain. The switch is based on the return for silage from dairies and the return of grain, which has been high the past couple of seasons.

“On the West Side of the San Joaquin Valley where there are fewer dairies, more of the wheat will be harvested for grain,” he said.

Grain yields have slowly been increasing with improved varieties. Yields of 4 to 4.5 tons are above the average, but not unusual today. In the days of widely planted Yecora Rojo, 3 to 3.5 tons were considered exceptional yields.

One of the more popular, newer wheat varieties has been Joaquin. It was a very close second to PR 1404, the most widely planted red or white wheat. Both were separated by a fraction at basically 21 percent apiece.

“Joaquin has been very popular because it makes good yields and good protein,” said Wright. “It has done its job, but for the past two years the amount of stripe rust found in Joaquin fields is increasing, many after fungicides were applied.”

Stripe rust has been around awhile, but it reared its ugly head in California in 2003 in epidemic proportions. It virtually eliminated Yecora Rojo from the California wheat picture. At one time 80 percent of the state was planted to that variety. This year it is less than 3 percent.

“It hit us in the face that year, and yields were down 50 percent or more that year from stripe rust. It hit some of the best varieties we thought were resistant.” By 2005, another bad year for stripe rust, California wheat growers lost another very popular variety, Blanca Grande, because of its susceptibility to stripe rust.

“There is so much stripe rust inoculum moving around with storms; it covered the state” he said. Wright said Lee Jackson, retired UC Davis wheat expert, has identified at least 19 different races of stripe rust, as he evaluates wheat trials around the state for stripe rust.

Worldwide epidemic

“Stripe rust is a worldwide epidemic,” said Wright, that often renders newer, more resistant varieties unwanted after just a few years because of the often rapidly growing susceptibility to stripe rust.

Earlier this spring Jackson reported “much” commercial acreage of Joaquin has been severely affected, even in instances when two fungicide applications were made.

Stripe rust has spawned considerable controversy. The university’s recommended approach in dealing with the problem is host plant resistance. Commercial seed companies say fungicides can keep a variety viable longer. Wright admits that growers can justify fungicide treatments on high yield, high quality wheat susceptible to stripe rust in these times of high grain prices.

“This year is when that happened with certain varieties” like Joaquin, Wright said.

Although breeders are rapidly developing stripe rust resistant varieties, Wright admits there is a “short list” for now of substitutes for the high yielding varieties susceptible to stripe rust.

However, the list is likely to get longer due to the work of UC Davis wheat breeder Jorge Dubcovsky who has identified a combination of stripe rust resistant genes called Yr5/Yr15 he has successfully introduced into wheat varieties.

Jackson reported that several new varieties containing the Yr5/Yr15 combination of genes were largely unaffected in the nurseries, where he found high levels of stripe rust in other varieties.

At a recent UC Davis wheat field day, Wright said Dubcovsky displayed a new Blanca Grande prodigy with the Yr5/Yr15 gene giving it stripe rust resistance. This could bring back the popular variety.

“It is a major breakthrough — a big change in the way we deal with stripe rust,” Wright said.

Gene breakthrough

Data from the stripe rust survey from 2010 shows that 17 races of the stripe rust pathogen were identified from 66 collections from California last season. None of these races can overcome the resistance of either Yr5 or Yr15.

Dubcovsky, who leads the UC Davis Wheat Breeding and Wheat Molecular Genetics Laboratories, made his breakthrough discover with a 2006 UC Discovery Grant in partnership with the California Wheat Commission.

The UC Discovery Grant promotes collaborations between University of California researchers and industry partners in the interest of supporting cutting-edge research. UCDG research projects are jointly funded by a UC Discovery Grant and an industry sponsor, like the California Wheat Commission, with a matching contribution.

Using this grant, Dubcovsky identified genes and transferred them into several California wheat varieties. "Without programs like the UC Discovery Grant, this research and knowledge would never get out of the laboratory," said Dubcovsky. "Now, we can transfer and advance the research so it benefits growers and consumers."

With a second UC Discovery Grant, Dubcovsky is partnering with the wheat commission to make wheat more hearty and nutritious, but also to maintain its long-term productivity and improve the market value of California wheat.

He was part of a team that cloned a gene that increases protein, iron and zinc in wheat, which provides about 20 percent of all the calories people consume worldwide. This June, he reported studies that could lead to a strain of wheat that could better tolerate freezing temperatures.

With the new discovery grant, Dubcovsky will focus on improving the nutritional value of California pasta and bread wheat, including increasing the concentration of resistant starch in the grain. This could improve dietary fiber and perhaps even make eaters feel full and cut down on overall calorie intake. New research also hopes to increase the grain protein concentration in wheat and discover new high grain protein genes that can help breed better varieties of wheat.

"Constant research is needed to stay ahead of disease that may harm crops, and we need to keep up with consumer demands for new, better and more nutritious varieties," said Lawrence Hunn, chair of the California Wheat Commission. The commission, funded by wheat growers, supports research to improve wheat quality and market development to provide growers with more outlets.

"Our partnership with UC researchers has always been critical to the viability of the wheat industry in California," said Hunn.

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