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Incidence, Transmissibility, and Genotype Analysis of Citrus tristeza virus (CTV) Isolates from CTV Eradicative and Noneradicative Districts in Central California

Growers in 45% (44,100 ha) of the citrus acreage in California stopped eradicating Citrus tristeza virus (CTV)-infected trees from their fields in 1995-96. The impact of leaving infected trees on the rate of CTV spread was determined by comparing temporal incidence of CTV in plots in Strathmore, Tulare County without eradication with incidence in a plot in McFarland, Kern County with eradication. From 1997 to 2003, CTV incidence in the Strathmore plots ranged from 6 to 42%, with annual spread rates from 1.6 to 3.6%. CTV incidence in the McFarland plot increased from 0 to 5% between 2001 and 2003 before infected trees were removed. Using a subplot hierarchical bulk sampling method, virus incidence over a 3-year period in a 6.5 km² area near McFarland was estimated to range from 0.09 to 0.69%, which indicated that CTV suppression was still being achieved in this area. Vector tests using the cotton aphid, Aphis gossypii, identified highly transmissible isolates (30 to 61% transmission rate) and a larger proportion of highly transmissible isolates were found in the McFarland plots. Thirty-six CTV isolates from recently infected plot trees were obtained and analyzed. None of these isolates reacted with monoclonal antibody MCA13 that detects presumptive CTV severe strains. Molecular analysis using polymerase chain reaction and sequence-specific primers showed that all isolates had a genotype identical to the T30 mild isolate from Florida.