Citrus rootstock topic of field day

Two severe citrus diseases that are currently not found in California - citrus variegated chlorosis and citrus blight - were the subjects of a presentation by UC Riverside plant pathologist Lawrence Marais at a field day in late March at the UC Lindcove Research and Extension Center.

Marias showed symptoms of citrus blight and citrus variegated chlorosis to enlist farmers' aid in monitoring for the diseases in California.

“In choosing a rootstock, it's always a compromise of advantages and disadvantages.”

CVC is now devastating the citrus industry in Brazil and Argentina. Spread by a sharpshooter that is similar to the glassy-winged sharpshooter, the incurable disease causes trees to decline and the fruit on them to remain small and very hard.

“The fruit cannot be processed at all,” Marais said. “The fruit is so hard it destroys the knives in the juice factory.”

The second disease, citrus blight, is now found in Florida, South Africa, Australia, Peru, Venezuela, Brazil and Argentina. It spreads through an orchard when infected tree roots graft underground with the roots of adjoining trees. The cause of the disease is unknown.

“The conductive tissues of the root systems and trunks become blocked with gum, resulting in impaired translocation of water and trace elements,” Marais said. “The trees start wilting, branches die back and the fruit remains very small indeed. The trees are unproductive within three years.”

Also during the field day program, UC Riverside geneticist Mikeal Roose took participants on a walking tour through two rootstock trials planted at Lindcove.

The first, planted 24 years ago by Roose's predecessor, compares Washington navel oranges on 19 different rootstocks. Scientists have collected data on the rootstocks' effects on yield, tree size, freeze tolerance, fruit quality and fruit size.

“As is normally the case when we're looking at a lot of characteristics, no one rootstock is superior for everything,” Roose said. “In choosing a rootstock, it's always a compromise of advantages and disadvantages.”

The second trial, planted in 1986, compares 21 rootstocks with Atwood navel scion. Many of these rootstocks have never been used in the San Joaquin Valley before, including several selections of Rangpur, the most important rootstock in Brazil. Some of the other rootstocks in this trial came from the USDA breeding program in India, which was closed in 1980.

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