|Yokomi, Raymond - Ray|
Submitted to: Society of Citrus Nurserymen International Congress
Publication Type: Proceedings
Publication Acceptance Date: 2/15/2014
Publication Date: 6/5/2014
Citation: Bederski, K., Silvestre, P., Roistacher, C., Giampetruzzi, A., Lonconsole, G., Hartung, J.S., Saponari, M., Yokomi, R.K. 2014. Thirty years of citrus tristeza virus observations in Peru. Society of Citrus Nurserymen International Congress. Available:http://iscn.co. Interpretive Summary: Citrus tristeza virus (CTV) causing CTV decline on sour orange and stem pitting regardless of rootstock devastated citrus production in Peru from the 1950’s. Therefore, in 1986, a citrus nursery located near Chincha, Peru, initiated cross-protection trials. CTV from symptomless trees were collected, inoculated into young citrus trees, and planted in an organic citrus field surrounded by severe CTV strains and aphid vectors. After 20+ years of evaluation, some CTV selections showed good control of CTV and were examined for molecular properties. Most of the protected trees contained both the severe and a mild strain of CTV. A few trees contained only the mild virus strain indicative of strain exclusion. The severe and mild strains were fully sequenced and identified. Sequences from protected and non-protected citrus suggested that multiplication of the severe CTV strain was down regulated in a manner consistent with gene silencing. In summary, these data showed long-term cross-protection for CTV may involve both RNA silencing and strain exclusion. These data will provide a better understanding of CTV cross protection that will facilitate identification and selection of CTV isolates to mitigate losses caused by CTV.
Technical Abstract: The Peruvian citrus industry was devastated by epidemics of Citrus tristeza virus (CTV) decline (CTV-D) on sour orange rootstock between 1950 and 1965 and CTV stem pitting (SP) between 1965 and 1985. CTV-SP debilitates citrus and fruit production regardless of rootstock. Control of CTV-SP by mild strain cross-protection has been shown to be effective and practical in other countries. From 1986, CTV isolates from symptomless trees in the coastal citrus-growing region of Peru as well as several other regions have been selected and field-tested continuously for cross-protection against SP and CTV-D symptoms in Chincha, Peru. Vigorous protected trees were kept while diseased trees were eliminated. After 20+ years of field trials, 20 isolates from cross-protected trees were selected and molecular properties determined. Eighteen of 20 isolates were comprised of severe strain CTV genotypes VT and/or T3 mixed with a mild T36NS genotype. Illumina next generation sequencing from three protected and one non-protected tree(s) were performed. Contigs from small interfering RNAs (siRNAs) were assembled and aligned against CTV sequences in GenBank. Two dominant variants were found: one was related to the severe NUagA SY strain from Japan; the other variant was related to the mild Poncirus trifoliata CTV resistance-breaking (RB) isolate from New Zealand. Sequencing and accumulation of the siRNAs derived from the severe and the mild strain showed a pattern consistent with putative RNA silencing where the severe strain was down regulated in the presence of the mild strain in protected trees. In addition, severe CTV strains were absent in the two trees where the putative protective isolate was a T36NS strain derived through passage in Citrus depressa cv. 'Shekwasha'. These data were consistent with exclusion of isolates of similar genotypes, a mode of action proposed for CTV cross-protection. However, the Shekwasha strain had a T36NS genotype whereas the Peruvian NUagA-like strain had a VT genotype. These data will provide an understanding of CTV strain interactions that will facilitate identification and selection of cross-protective isolates to mitigate losses caused by CTV.