|Kwon, S. J.|
|Yokomi, Raymond - Ray|
Submitted to: Frontiers in Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/18/2013
Publication Date: 12/10/2013
Citation: Wang, J., Bozan, O., Kwon, S., Rucker, T., Yokomi, R.K., Lee, R.F., Folimonova, S., Greer, G., Krueger, R., Bash, J., Vidalakis, G. 2013. Past and future of a century old Citrus Tristeza Virus collection: A California citrus germplasm tale . Frontiers in Microbiology. 4:366. DOI: 10.3389/fmicb.2013.00366. Interpretive Summary: Strains of Citrus tristeza virus (CTV) that cause quick decline (QD) of citrus grown on sour orange rootstock or severe stem pitting (SP) of sweet orange and/or grapefruit scions, regardless of rootstock used, are among the most damaging viral pathogens of citrus. Where CTV is endemic, growers avoid sour orange rootstock and use trifoliate orange or its hybrids which have CTV-resistance or tolerance. SP, however, is a scion disease and affects citrus regardless of rootstock. Moreover, susceptibility to SP varies among orange and grapefruit varieties. Because CTV is readily transmitted by aphids, in areas with prevalence of CTV-SP and aphid vectors, mild strain cross-protection is used to control damage from CTV-SP. Examples where cross-protection are used include Brazil and South Africa for Pera sweet orange and Marsh grapefruit, respectively. CTV cross-protection efficacy recently has been shown to be genotype dependent. Cross-protection between mild and severe CTV strains occur in the same genotype; superinfection (where a severe strain overcomes previous infection of a mild strain) occurs when CTV genotypes are different. Therefore, determination of the genetic diversity, genotype and virulence of CTV are critical in selection of mild strains for cross-protection. To this end, a study was undertaken using a collection of California CTV isolates maintained by the University of California’s Citrus Clonal Protection Program (CCPP), Riverside, CA. The CCPP provides a mechanism for introduction and distribution of citrus germplasm from any citrus-growing area of the world to California for use in research, variety improvement, and industry. The CCPP pathogen collection includes CTV isolates interdicted from field sources and introductions from as early as 1914. Genotypes of 48 CTV isolates from the CCPP collection were characterized by reverse transcription polymerase chain reaction amplification with sequence-specific primers for different CTV genome regions as well as sequencing of the major coat protein gene. The resultant molecular characterization was coupled with biological characterization of isolates on a set of citrus indicator hosts. The presence of CTV genotypes T30, T36, and VT were identified in single or mixed infections; however, no clear association was found between symptom severity and a specific genotype of CTV. This study provides a historical assessment of early CTV genetic diversity in California under the CCPP program. The CTV data will be useful for research efforts to improve diagnosis for economically important strains of CTV and disease management strategies including cross-protection.
Technical Abstract: The California Citrus Clonal Protection Program (CCPP), Riverside, CA provides a mechanism for introduction and distribution of citrus germplasm from any citrus-growing area of the world to California for use in research, variety improvement, or by industry. Citrus tristeza virus (CTV) is a serious citrus pathogen worldwide. The presence of CTV in introduced citrus germplasm as well as in dooryard and field trees in California has been determined and documented for decades at the CCPP. Representative CTV isolates have been maintained in planta under quarantine in the CCPP disease bank from germplasm introductions and field sources dating back to 1914. Genotypes of 48 CTV isolates from this collection were characterized by reverse transcription polymerase chain reaction amplification with multiple molecular markers targeting different CTV genome regions, followed by sequencing of the major coat protein gene. Molecular characterization of isolates was coupled with biological characterization using a set of citrus indicator hosts. The presence of T30, T36, and VT genotypes of CTV was identified in single or mixed infections, however, no clear association was found between severity of symptom expression and a particular CTV genotype. This study provides a historical assessment of early CTV genetic diversity in California under the prism of a germplasm program that created CTV-free scion sources for the California citrus industry. These data will be useful for research efforts on development of CTV management strategies such as cross-protection and further genetic characterization of CTV in California.