|Yokomi, Raymond - Ray|
Submitted to: Proceedings of the Conference of the International Organizaion of Citrus Vi
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/19/2011
Publication Date: 12/10/2011
Citation: Yokomi, R., Saponari, M., Metheney, P., Vidalakis, G. 2011. Genetic differentiation and biology of Citrus tristeza virus populations spreading in California. In: Proceedings of the 18th Conference of the International Organizaion of Citrus Virologists. Available:http://iocv.org/proceedings/eighteen/Yokomi_et_al.pdf. Interpretive Summary: Citrus trees infected with Citrus tristeza virus (CTV) in Tulare, Kern, Ventura, Riverside and San Diego Counties were sampled. Laboratory tests to assess molecular and biological properties of the CTV isolates with regard to diversity, severity and identification were performed. Analyses indicated typical isolates from central California had a genetic profile associated with mild strains and, in greenhouse virus index tests, induced mild symptoms. Virus isolates from southern California and a few from central California were mostly the same mild strains but some were also infected with strains reactive to MCA13 monoclonal antibody that is known to react with severe strains that cause decline of citrus planted on sour orange rootstock. MCA13 also reacts with other highly virulent CTV strains but, by itself, is not an indicator of virulence. MCA13-reactive isolates were further tested with a PCR assay which targets: i) a marker gene sequence associated with mild strains called T36 non-standard (NS); or ii) a marker gene sequence called CPiVT3 which identifies severe CTV strains with a genomic profile called VT or T3. The CPiVT3 probe readily identified isolates with a T3 genetic profile. Isolates representing both marker groups were evaluated for severity in greenhouse index tests. Isolates with the T36NS profile produced mild symptoms; whereas those with the T3 genetic profile produced severe symptoms. Thus, California isolates tested fell into two biological groups: mild with a T30 and/or T36NS genotype; or severe with a T3 genotype. These data validate use of genotype-specific probes in field surveys to differentiate mild and severe CTV strains. This procedure has been adopted by the grower-supported Central California Tristeza Eradication Agency, Tulare, CA for the control of CTV in central California.
Technical Abstract: Citrus tristeza virus (CTV) isolates were collected from more than 1500 trees in citrus groves in Tulare, Kern, Ventura, Riverside and San Diego Counties for laboratory tests to assess molecular and biological properties of CTV strains currently in California. Tests included serology with MCA13 monoclonal antibody, quantitative reverse transcription (RT) polymerase chain reaction (PCR) assays with strain-specific markers, single stranded conformational polymorphism (SSCP) analysis of amplified products from the coat protein (CP) gene region and sequencing of PCR products from the CP and P20 genes. Isolates from central California typically had a T30 genotype and were mild or induced no symptoms on indicator plants. Isolates from southern California and a few from sites in central California were mostly T30 strains in single or mixed infections with a non-standard (NS) and/or T3-like strain. Phylogenetic analysis of the CP and P20 gene sequences showed that NS strains clustered in the same main clade as T36 and were named T36NS. A genetically diverse set of isolates were selected and indexed in greenhouse tests. T36NS strains reacted with MCA13 and were mild on Mexican lime and symptomless in other citrus indicators. T3-like strains in single or mixed infections with T30 were also MCA13 positive and induced strong seedling yellows in indicator plants. In general, the California isolates tested fell into two biological groups: mild with a T30 and/or T36NS genotype; and a severe SY strain with a T3 genotype. These data support validation of genotype-specific probes used in field surveys to differentiate putative economic CTV strains.