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Economic impact of mild tristeza virus strains on tolerant rootstocks

The Issue

Citrus tristeza is among the most serious viral diseases of citrus worldwide. The virus resulted in the loss of 3 million orange trees on sour orange rootstock in Southern California during the 1940s and 1950s. In light of this impact on the California citrus industry, in 1963 a tristeza eradication agency was established in the San Joaquin Valley to survey, detect and remove commercial citrus trees infected with citrus tristeza virus. Five pest control districts were established within the agency. Early survey and laboratory screening by the eradication agency generally detected mild strains of the virus. In 1996, two districts withdrew from the eradication program convinced that the virus was not seriously harming infected trees. The decision was based on the fact that commercial orchards in the districts were generally grown on rootstocks thought to be tolerant of mild strains of the virus.

What Has ANR Done?

A research trial was established to measure the impact of the virus in commercial citrus orchards grown on rootstocks thought to be tolerant of mild strains of the virus. Ten commercial navel orange orchards were selected within the Tulare County Pest Control District. Using Central California Tristeza Eradication Agency records, orchards with approximately 10 percent infection were selected and approximately 200 data trees selected in each orchard, including trees that were positive and negative for the virus. Trees were harvested individually to determine fruit weight, count, size and grade. Annual canopy measurements were recorded. Relative health scores were developed for each tree and collected seasonally for each orchard. Trees were sampled annually for the presence of tristeza by technicians. The agency conducted bio-characterization studies on isolates from the orchards and found that these strains were mild.

The Payoff

Production, fruit quality unchanged in infected trees

An analysis of the data was conducted in collaboration with a team from the Craig School of Business at California State University, Fresno. No significant difference in tree health was measured between infected and non-infected trees and fruit values from trees with the virus were equal to values from non-infected trees. With the results of this research, growers have scientific evidence that trees on tolerant rootstocks that are infected with a mild strain of citrus tristeza virus will not collapse like the trees in Southern California did half a century ago. However, they must weigh that fact with the knowledge that maintaining infected trees in the orchard increases the risk that citrus tristeza virus will continue to spread from tree to tree in the orchard and in the region.


Supporting Unit: Tulare County

Neil O'Connell, (559) 684-3312, nvoconnell@ucdavis.edu