New mildews in melons bring calls for breeder help

Outbreaks of new forms of powdery mildew in California and Arizona desert melon fields this and the past two seasons sent the call for tougher varieties to resist the fungal threat.

At work to find new germplasm from exotic cucurbit plants for breeding into improved varieties is James McCreight, research leader and geneticist at USDA's Agricultural Research Station at Salinas.

McCreight, who's been at the station since 1979 and spent the two previous seasons at a USDA facility in the Imperial Valley, is no stranger to powdery mildew.

Yet he is the first to admit that he and his colleagues need to know more about how the disease works against cantaloupes, honeydews, muskmelons and other melons.

Ten or so races of powdery mildew are known to exist around the world, but only one race was thought, until recently, to be in California, and a second was known to be in Texas.

Melon trials in the desert in 2002 by Tom Turini, Imperial Valley farm advisor, showed significant Race I pressure. Then in the 2003 season, Race II showed up to create major infections in commercial melon varieties previously thought to be resistant.

Race II infections were later found at Yuma and in experimental plots at the Salinas station. At about the same time, evidence emerged pointing to the new race challenging fungicides.

Confirms Race II presence

Michael Coffey, plant pathologist at the University of California, Riverside, collected several samples of the disease for laboratory evaluation and confirmed that Race II was in California fields.

“The fortuitous thing,” McCreight said, “is we had been working with a melon from India originally selected for its resistance to lettuce infectious yellows virus. Our research shows it is the only melon that has resistance to Race II.”

Powdery mildew was back in Imperial Valley melons again this spring, but so were Turini and McCreight, collaborating on trials observing plant material from the Indian melon, whose powdery mildew resistance comes from a trio of genes. Data from the trials are being collected for analysis later this year.

“So far,” McCreight said, “the preliminary data suggest there may be a different race there this year. It may be any one of the 10 we know about, or it may be something different.”

Growers, he said, are attracted to resistant varieties because they supposedly preclude the need for fungicides. However, he added, it's only a matter of time before the disease changes to overcome the resistance in a new variety.

Works on other fronts

“We really don't know much about the genetics of the pathogen. We haven't seen a sexual stage for the disease, so it may be chance mutations causing the new forms.”

McCreight also works on other fronts to improve cultivars for the California melon industry. Cucurbit leaf curl virus is a whitefly-vectored disease that has appeared here and there in desert melons for the last 10 years without serious consequences.

He added, however, “We found a beautiful, uniform infection of what we think is it in Imperial Valley fields last year, so were are confirming that in the greenhouse this year.”

It turns out the same Indian melon carrying powdery mildew resistance also shows resistance to the virus.

Yet another dimension of his research is a joint project with USDA geneticists at the University of Wisconsin on melon plants that have a “multiple lateral branching” trait. Commercial varieties in the U.S., in contrast, extend in a linear fashion with fewer branches.

“Ideally, with this type of plant,” he said, “you could set several fruit almost simultaneously for a once-over harvest. But that is still several years off.”

A related project is investigating development of melon plants having more female flowers and increased yields.

McCreight allots 60 percent of his time to melon research and the remainder goes to lettuce projects. One example of his work in lettuce is a continuing search for resistance to Fusarium wilt, which was detected in certain fields in Yuma in 2001 and around Huron in 2002 and has since spread.

Fusarium wilt resistance

“We've been characterizing the races of it we have, based on samples of it from Japan. It's been on lettuce there since 1955 and there are now three races there. So far, our data suggest we have only one, Race I, the original they had in Japan.”

His aim is to find germplasm to breed Fusarium wilt resistance into varieties for the fall season in Yuma. Japanese breeders found resistance in the California variety Salinas, which is typically unaffected by the disease due to cooler conditions on the California coast but is not adapted to the desert climate.

Mike Matheron, University of Arizona research scientist, is currently running field screening of commercial varieties to determine how they fare under pressure from the disease.

Eventual improved commercial varieties need not only resistance to the wilt but heat tolerance along with resistance to bolting and tipburn.

McCreight also supervises the other projects at the station, including work by a dozen USDA scientists and one IR-4 Program researcher involved with their respective specialties of vegetable breeding, sugar beets, insect and virus control, soilborne diseases, postharvest insect control, and organic vegetables. The station budget is $4.1 million per year.

He also oversees the collaborative work of three University of California, Davis scientists and one Artichoke Research Association entomologist attached to the station.

The station was established in the early 1940s for strategic research on production of guayule, a rubber substitute. Sugar beet research came in 1948 and in 1956 vegetable research was added. It has 37 buildings, and 33 greenhouses on 13 acres, plus three field research locations totaling nearly 200 acres.

USDA, McCreight said, allocated $4.5 million this year for design and planning of modernized and expanded facilities housing up to 25 scientists at the site. Planning was set to begin in July.

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