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Powdery mildew in lettuce under study

USDA plant breeders have identified 24 lettuce cultivars having resistance to powdery mildew, a mounting problem in lettuce grown in the California-Arizona desert during the late winter and early spring period.

During the Annual Desert Crops Workshop at Yuma, Ed Ryder, geneticist at the USDA Station in Salinas, Calif., said 10 cultivars show high resistance, while another 14 show possible moderate resistance. Nearly 60 cultivars, including several old butterhead varieties, were tested in Yuma trials.

Crosses to transfer the resistance were made during 2002 and selections continue in 2003 in the joint project with university and industry resources of the two states.

Ryder said selections have been made in Yuma from among crosses of resistant cultivars Clarion, Saraya, and Big Boston and the desert cultivars Winterhaven, an iceberg type, and Darkland, a romaine type.

Backcrosses with Winterhaven and Darkland will be made to obtain desired respective traits of the two types, plus powdery mildew resistances.

Big vein resistance

He also reported progress in achieving resistance to big vein, a mid-winter problem for desert lettuce and a spring problem on the California coast. Earlier work in Salinas produced resistant cultivars with heritage of the wild and highly resistant Lactuca virosa.

One of those, Pacific, was crossed with Winterhaven to achieve plants having resistance and Winterhaven characteristics.

The breeding also turned to improving resistance to tipburn, common in the late winter and early spring in the desert. “A number of years ago we released a variety called Tiber, which has high resistance. It is primarily a coastal type and is essentially unsuitable for the Yuma area,” Ryder said.

“So in order to develop a relatively closed-head variety suited to the desert, we went back to the original cross, Salinas 88 with Vanguard 75, that gave us Tiber, and planted that material at Yuma. We got four families with the desired head type and tipburn resistance.”

The material, he added, showed a 10 percent susceptibility in contrast to a range of 75 to 90 percent susceptibility in commercial varieties.


Collaborating with Ryder in the projects are his USDA colleagues in Salinas: Rebecca C. Grube, Beiquan Mou, and James McCreight.

In a poster display on his Palo Verde Valley trials with dehydrator onions, Michael D. Rethwisch, Riverside County farm advisor, said a single application of AuxiGro WP, a gamma aminobutyric acid compound, improved yield by about 0.7 ton per acre.

Two applications increased yield of low desert-grown onions by 1.3 tons per acre. “As dehydrator onions are worth approximately $100 per ton,” Rethwisch said, “it appears that usage of AuxiGro WP will be economical as two, 4-ounce per acre applications are expected to cost about $35, indicating a positive economic return of approximately $95 per acre.”

Three applications

His trial consisted of three applications: an early, single, 4-ounce treatment on April 16, or 52 days before harvest; a late, single, 4-ounce treatment on May 10, or 28 days before harvest; and a combination of both treatments on the same plot.

Also known as a GABA, the compound promotes movement of calcium and potassium in plants. AuxiGro, the only product to carry the GABA chemistry, has EPA registration for fresh market and storage onions (but not processing/dehydrator types), various vegetables, field crops, trees and grapes, and grass seed.

Its California registration is limited to almonds, disease control and soluble solid increase on grapes, fresh market and storage onions, and processing tomatoes.

The annual workshop, attended by more than 100 persons at Yuma's Civic & Convention Center, was jointly sponsored by Western Farm Press, University of Arizona Cooperative Extension, and University of California Cooperative Extension.

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