Within weeks, scientists will begin moving 600 disease-free citrus trees into a 20,000-square-foot screenhouse at the UC Lindcove Research and Extension Center near Exeter, protecting them from aphids that transmit the devastating citrus tristeza virus.

The $360,000 project -- funded entirely by the citrus industry -- includes a 2,300-square-foot laboratory building.

Potted trees growing in the screenhouse will be nursed by scientists, who will prune them for maximum budwood production and feed them carefully dosed fertilizer every day with their irrigation water. A systematic pest control routine will be undertaken. The trees will be repotted into successively larger containers until they are in four-foot-square crates.

"Container-grown trees require special care to keep them in good health," said Plant Pathologist David Gumpf, director of the UC Riverside Citrus Clonal Protection Program.

The screenhouse trees will eventually replace the 14-acre Foundation Block, a collection of 200 varieties of citrus trees growing in a field at Lindcove since 1961. The open-air trees are now threatened by the citrus tristeza virus, which can be transmitted by aphids from diseased trees surrounding the research station.

Last fall, four trees at the Lindcove Research and Extension Center tested positive for tristeza. None of the four was in the Foundation Block and all were removed and destroyed. Trees at the station are again being indexed for tristeza and results will be available shortly.

The Foundation Block is California's primary source of true-to-type, virus-tested budwood. Nurseries use the budwood to grow new citrus trees for farmers. Nearly all of California's citrus trees are propagated from the Lindcove Foundation Block, including traditional valencia and naval oranges, grapefruits, lemons and limes, plus exotic citrus fruit, like blood oranges, acid-free oranges, cantaloupe-sized pummelos, citrons, tangors, kumquats and sweet lemons.

The citrus tristeza virus was first detected in San Joaquin Valley orchards about 40 years ago. An aggressive eradication program was successful in keeping the disease at very low levels. However, because of concerns about the costs associated with removing infected trees, a petition led to a moratorium on mandatory tree destruction and a voluntary program was initiated.

"Eventually, the disease pressure around the station will be so great the trees in the existing Foundation Block will become infected," Gumpf said.

However, he said he is hoping to use the existing Foundation Block for a while longer.

"I'd like to wait two to three years before we begin cutting budwood from the screenhouse trees," he said. "They need a chance to grow."

The screenhouse project was conceived by a subcommittee of the Citrus Research Board, a grower-funded, grower-directed state marketing order program. The CRB protects the economic viability of the California citrus industry by supporting activities in the areas of quality assurance, production and variety research, and grower and public education.

The new facility, located on a five-acre field in the northeast corner of the Lindcove Research and Extension Center, will be managed and maintained by the University of California with yearly operational funds from the Citrus Research Board.

Jeannette Warnert
Public Information Representative
Ag and Natural Resources
UC Center
550 E. Shaw Avenue
Fresno, CA 93710