Submitted to: Plant Disease
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/5/2005
Publication Date: 8/1/2005
Citation: Yokomi, R.K., Deborde, R.L. 2005. Incidence, transmissibility, and genotype of citrus tristeza virus (CTV) isolates from a CTV eradicative and a non-eradicative district in central california. Plant Disease. 89(8):859-866. Interpretive Summary: The incidence of citrus tristeza virus (CTV)-infected trees in plots with and without CTV eradication in central California were compared from 1997 to 2003. Disease incidence in plots without eradication ranged from 6 to 42% with annual increases from 1.6 to 3.6%. Disease incidence in a 442-tree plot in an eradication district went from 0 to 5% in four years and demonstrated the invasiveness of the disease agent. This lead to a separate CTV survey conducted from 2001 to 2003 in a nearby 727 ha of citrus within the eradication district. CTV incidence here was estimated to range from 0.2 to 0.7% which indicated that CTV suppression was still being achieved. Field trees infected with CTV were symptomless. Molecular analysis showed that all CTV isolates collected had a genotype identical to the T30 mild isolate from Florida and none of the isolates reacted with MCA 13, a severe strain discriminating monoclonal antibody. Vector tests using the cotton aphid, Aphis gossypii, separated the CTV isolates into categories of low (0 to 15%) and high (30 to 61%) transmissibilities. These results indicate that CTV is still under control in eradication districts and most CTV isolates are comparatively mild and range from low to high levels of aphid transmission.
Technical Abstract: In 1996, growers in 44,100 ha (45%) of citrus in central California left the citrus tristeza virus (CTV) eradication program and no longer remove CTV-infected trees from their groves. The impact of this on CTV epidemiology and the eradication program was determined by annual surveys of temporal CTV incidence in plots located within and outside of CTV eradication districts. Disease incidence in mature plots outside of the eradication district ranged from 17 to 42% and rate of increase was linear from 1.6 to 3.6%. This suggests that much of the spread was primary not secondary, and that the major source of vector populations were from non-citrus host. The annual disease increase in young citrus inside and outside of eradication districts were found to be similar. Because this is troubling, CTV spread over a larger (6.5sq Km2) area under eradication was surveyed over a 3-year period to determine if control was still being achieved. Estimated incidence was found to average between 0.09% and 0.69% indicating that CTV was still under suppression. Vector studies with the cotton aphid revealed that highly transmissible isolates (30% to 61% transmission by 5 to 10 aphids) occur and that a greater portion of these highly transmissible isolates were found in eradication versus non-eradication areas. Using the multiple molecular marker method and RT-PCR, all plot isolates had a T30-like genotype and did not react with the severe-strain discriminating monoclonal antibody MCA13. This suggests that although isolates are being spread in central California by endemic aphid vectors, these virus strains have a similar ancestry to the T30 mild isolate from Florida and pose less potential risk than if other genotypes were present.