Aphid resistance gains in USDA lettuce trials

Lettuce growers in California's Salinas Valley must battle the lettuce aphid for the foreseeable future, but USDA scientists say they have new plants to stop the hard-to-reach pest in its tracks.

Relief won't come overnight. In fact, it will be “several years” before lettuce plants resistant to the aphid are turned over to commercial seed companies to develop as new varieties, according to Edward Ryder, plant geneticist at the USDA Salinas Station.

“We have obtained from Europe material that is highly resistant to the lettuce aphid, and we'll be using it to develop plants that are most suitable for the western U.S. The resistance comes from a single gene.”

Ryder, who collaborated with Bill Chaney, Monterey County farm advisor, in tests, says lettuce aphids placed on susceptible lettuce plants set up housekeeping in the interior of the heads, safe from insecticide sprays, and reproduce profusely.

“However,” Ryder says, “when they are placed on resistant lettuce plants, they simply don't thrive and disappear. It's some sort of anti-biosis process at work.”

His plan is to transfer the resistant gene, through a series of crosses using classical breeding techniques, from a European commercial variety first to iceberg and later to romaine lettuce. Crosses are under way at the station.

The protracted process is partly because European lettuce varieties tend to have considerably smaller heads than those common in the U.S. The objective is to gain the resistance without sacrificing marketability here.

Wait several years

Declining to commit to a deadline, Ryder, who did the breeding for the popular “Salinas” variety and other commercial varieties, concedes it will take several years to accomplish, although not as many as if the aphid-resistant gene were in a wild lettuce.

That's not the only deadline he is coy about. At the station since 1957, the same year he joined USDA, he once planned to retire in 1995. There was some talk about it again in 1998, but now he shows little inclination to relinquish his parking space or the pace of his projects.

Ryder and his fellow plant scientists at the station have much to do on several fronts for the lettuce industry. Another veteran, James McCreight, research leader at the station, specializes in melons and cucurbits and also does some lettuce breeding.

Becky Grube, a geneticist with a background in pepper viruses at Cornell University, came to the station about a year ago and works full-time in lettuce breeding.

Vegetable breeder Beiquan Mou, coming from a post-doctoral research assignment at Iowa State University, was due at the station in March. His time is equally divided between lettuce, spinach, and celery.

The team is working on several breeding projects, some new, others continuing, under support from the California Lettuce Research Board. Certain other projects receive funds from the Arizona Iceberg Lettuce Research Council.

Ryder makes selections from resistant plants at the station greenhouse and then joins forces with farm advisors and others for evaluations of the plants in commercial fields in Monterey County and the Yuma area.

“We've added several new projects as a result of the board functions taking in all lettuce rather than only iceberg lettuce. We are working on many mixed-lettuce problems which are, in many respects, the same, but we also have projects to find resistance for newly emerged problems,” Ryder said.

The priorities include Verticillium dahliae, a soilborne fungus new to lettuce but quite damaging to the crop. It is expected to be a persistent problem.

Tomato bushy-stunt virus, also known as lettuce dieback, damages romaine and leaf lettuces and reappeared along the Salinas River after flooding in 1998. Some sources of resistance have been located and crosses have been made.

Ryder is cooperating with farm advisors in Monterey County and Imperial County in a successful search for cultivars having resistance to powdery mildew.

Crown rot in romaine

Also getting attention of the breeders is crown rot, a new disease of romaine which some fear might go to other lettuce varieties. It showed up in a limited area in Monterey County during last spring, but research progress was thwarted when the disease failed to occur in the fall.

The industry and scientists of USDA and the University of California, mindful crown rot might be sporadic like anthracnose, remain alert.

Work to find resistance to downy mildew is expanding. The Salinas USDA team is monitoring for multiple-gene “field resistance” while molecular geneticists at UC, Davis pursue single-gene resistance.

In keeping with the broader mission of the lettuce research board, Ryder has been working to intensify the color of green leaf varieties.

“The older varieties have a light yellowish-green color, but dark green is usually favored by consumers,” said Ryder. Related work may be done to increase the color of red leaf varieties.

Resistance to insect and disease pests of lettuce is collected literally from around the world and is represented in the USDA lettuce germplasm collection, one of the largest of its type in the world, at the Salinas station.

It has nearly 11,000 items, including more than 5,000 breeding materials, about 1,400 cultivars, 1,400 plant introductions, and 2,800 items of molecular markers and transgenic materials.

Ryder, as curator, maintains the collection in a humidity-controlled freezer where seeds in coin envelopes remain viable for upwards of 25 years. It is duplicated at the USDA station at Pullman, Wash., and portions of it are duplicated elsewhere. He periodically plants out seed and harvests for new supplies.

The collection has new and old commercial varieties, landrace plants native to certain locations or individual growers, and heirlooms from seed companies and breeders. New additions are constantly made.

And the old varieties sometimes more than justify the trouble of saving them. One Burpee Seed Co. variety dating from 1894 fell out of commercial use with the advent of varieties having larger heads and better shipping quality. The old variety, however, carried genes resistant to powdery mildew and sclerotinia and useful today for breeding.

The collection was begun by Ryder's predecessor at the station, Ross C. Thompson, in 1931 in Beltsville, Md., where some materials were used in breeding. In 1956 Thompson transferred to the Salinas station and brought the seeds with him.

Ryder said the collection has been particularly valuable in responding to the current emphasis on breeding materials having multiple resistances to lettuce mosaic, corky root, and tipburn in the same accessions. It enables breeders to make rapid progress against several problems at once.

Outlook for 2010?

Asked what he perceived the line-up of commercial lettuce varieties for California will look like in 2010, Ryder replied: “If the present trends continue, a number of things will probably happen:

“The number of cultivars in the romaine, leaf lettuce, and butterhead group will increase, along with acreage. Mixed lettuce now makes up about 40 percent of the production and is rising.”

Another will be varieties bred for the boom in cut lettuce. “You can use a different configuration of head for chopped lettuce, with fewer wrapper leaves, compared to lettuce marketed as a whole head.”

Attention to cosmetics of fresh produce, driven by retailing in see-through-packaging, will become increasingly important. Breeding will attempt to minimize cosmetic losses to tipburn, caused by calcium deficiency at the outer edges of leaves during rapid growth, he said.

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