No abstract

Biological indexing for graft-transmissible pathogens of citrus in the presence of additional pathogens was investigated. The probability for symptom expression, the efficacy of the bio-indexing tests, and the number of citrus indicators required for pathogen detection were statistically evaluated. Multiple infections did not preclude symptom expression or reduce the diagnostic efficacy of the primary indexing hosts for Citrus tristeza virus (CTV), Citrus psorosis virus (CPsV), and Citrus tatter leaf virus (Apple stem grooving virus). Symptoms of Citrus vein enation virus (CVEV) and the diagnostic efficacy of Mexican lime were suppressed by the T30 group CTV isolates, but not by other CTV isolates tested. CPsV suppressed symptom expression and diagnostic efficiency of Dweet tangor and sweet orange for concave gum. The application of alternate bioassay hosts for indexing was also investigated. Dweet tangor, sweet orange, and Citrus excelsa are not typically used for bioindexing of CVEV, however, Dweet tangor and C. excelsa detected CVEV in single infections, whereas in sweet orange, CVEV was detected only when CPsV, concave gum, or citrus viroids were present. CTV was readily detected using the alternative indicator C. excelsa, whereas only shock reacting CPsV isolates were effectively indexed by Mexican lime.

The term 'transmissible small nuclear ribonucleic acids' (TsnRNAs) describes well-characterised viroid RNA species that do not induce any pronounced disease syndromes in specific citrus hosts, but rather act as regulatory genetic elements modifying tree performance. The canopy volume (CV) of 13-yearold navel orange trees (Citrus sinensis) on Poncirus trifoliata rootstock treated with TsnRNA-IIIb (syn. Citrus dwarfing viroid) was reduced by 45% and 53.5% in standard-density (6 m × 6.7 m) or high-density (3 m × 6.7 m) plantings, respectively. The total yield of eight consecutive harvests was not affected significantly by the TsnRNA-IIIb treatments or the two planting densities. However, the yield per land surface unit (Y/LSU) was almost doubled (increased by 97.5%) for the high-density plantings over the standard-density plantings of the untreated controls. The Y/LSU of the TsnRNA-IIIb treated navel orange trees in the standard-density planting was reduced by 32.7%. The TsnRNA-IIIb treatment in both planting densities concentrated significantly more fruit production (approximately 60%) in the economically advantageous middle canopy height zone (0.6-2.4 m) in comparison with the untreated controls (35%). Fruit grade, size, appearance, organoleptic characteristics or time of maturation of the TsnRNA-IIIb dwarfed navel trees were not significantly different between the two planting densities and the controls. Fruit with higher commercial value was produced in the TsnRNA-IIIb dwarfed navel trees in the high-density planting by 3.9% and 4.6% over the TsnRNA-IIIb or controls in standard-density planting respectively. The increase in Y/LSU and fruit value for the TsnRNA-IIIb treated navel trees in the high-density plantings in combination with the reduced management cost of dwarfed trees could result in substantial higher profits for a commercial grove despite the higher establishment cost of high-density plantings.

Citrus tristeza virus (CTV) isolates collected from citrus germplasm, dooryard and field trees in California from 1914 have been maintained in planta under quarantine in the Citrus Clonal Protection Program (CCPP), Riverside, California. This collection, therefore, represents populations of CTV isolates obtained over time and space in California. To determine CTV genetic diversity in this context, genotypes of CTV isolates from the CCPP collection were characterized using multiple molecular markers (MMM). Genotypes T30, VT, and T36 were found at high frequencies with T30 and T30+VT genotypes being the most abundant. The MMM analysis did not identify T3 and B165/T68 genotypes; however, biological and phylogenetic analysis suggested some relationships of CCPP CTV isolates with these two genotypes. Phylogenetic analysis of the CTV coat protein (CP) gene sequences classified the tested isolates into seven distinct clades. Five clades were in association with the standard CTV genotypes T30, T36, T3, VT, and B165/T68. The remaining two identified clades were not related to any standard CTV genotypes. Spatiotemporal analysis indicated a trend of reduced genotype and phylogenetic diversity as well as virulence from southern California (SC) at early (1907–1957) in comparison to that of central California (CC) isolates collected from later (1957–2009) time periods. CTV biological characterization also indicated a reduced number and less virulent stem pitting (SP) CTV isolates compared to seedling yellows isolates introduced to California. This data provides a historical insight of the introduction, movement, and genetic diversity of CTV in California and provides genetic and biological information useful for CTV quarantine, eradication, and disease management strategies such as CTV-SP cross protection.

The unusual symptom, “finger imprint”, described exclusively on Poncirus trifoliata, has been reported in only a single field trial investigating the effects of citrus viroids on crop performance. With this, the question has persisted whether the observed growth abnormality was a disease symptom induced by Citrus viroid IIIb (CVd-IIIb) or a consequence of mechanical damage caused by the handling of young trees during propagation or cultural practices in the field. The recurrence of finger imprint symptoms on trees after 5 years in the field in which no abnormal growth features were previously noted now supports the proposition of a viroid-induced disease. The symptom expression results from an unusual etiology of a complex relationship of the specific viroid CVd-IIIb on the specific rootstock P. trifoliata only when supplemental water is applied by sprinkler irrigation.

The term ‘transmissible small nuclear ribonucleic acids' (TsnRNAs) describes well characterised viroid RNA species that do not induce any disease syndromes in specific citrus hosts but rather act as regulatory genetic elements modifying tree performance. Twelve‐year‐old navel orange and 10‐year‐old Clementine mandarin trees on Carrizo citrange (Citrus sinensis×Poncirus trifoliata) rootstock treated with a mixture of three TsnRNAs (−Ia, syn. Citrus bent leaf viroid, +IIa, syn. Hop stunt viroid and +IIIb, syn. Citrus dwarfing viroid) were reduced in size by 33% and 43%, respectively. Clementine trees treated with a mixture of TsnRNA−Ia+IIa or −Ia+IIIb also had reduced canopy volume (CV) (∼38 and 31%, respectively), whereas trees treated with TsnRNA−IIa+IIIb showed little effect. The effects of the double TsnRNA treatments −Ia+IIa and −Ia+IIIb on Clementine canopy size and commercial performance were comparable and in some cases superior to that of the triple TsnRNA mixture. The TsnRNA−Ia+IIa treatment had the most attractive commercial traits with increased production of Clementine fruit per CV (23.6%), more fruit with high commercial value (31.7%), and more fruit optimally distributed in the canopy (68% of fruit between 0.5 and 2.5 m). None of the TsnRNA treatments affected the growth of Carrizo rootstock seedlings after 8 years in the field. Navel orange and Clementine scions treated with the same triple TsnRNA mixture expressed different trunk and fruit production patterns although effects on CV were similar.

SummarySystemic interactions of long duration among the Group I] Citrus Viroids, CVd‐IIa and CVd‐IIb were investigated by grafting buds from established citron (Citrus medica L.) sources containing single viroids to a common healthy seedling. In healthy tissues, the two viroids became established in approximately equal titres as a mixed infection. By contrast, tissues that grew from the CVd‐IIa or CVd‐IIb source buds contained only trace amounts of the heterologous invading viroids.This interference between CVd‐IIa, a mild exocortis agent, and CVd‐IIb, the cachexia agent, was also demonstrated in the presence of CEVd, the severe exocortis agent but not a Group II viroid. When a CVd‐IIb bud was propagated on a citron containing CVd‐IIa, a dramatic reduction in the titre of CVd‐IIb was observed.The interference between CVd‐IIa and CVd‐IIb indicates that the mild and relatively innocuous isolate, CVd‐IIa, can interfere with the replication and/or accumulation of the severe cachexia agent, CVd‐IIb, in citrus. This offers a potential practical approach for the control of cachexia in commercial plantings by “viroid interference”.

Observations on a Valencia/trifoliate planting inoculated with pure isolates of citrus viroids (CVds) showed the following specific reactions: 1) "Trifoliate deep pit" a symptom caused by citrus viroid Ia which induced deep pits in the wood of the trifoliate trunk with corresponding pointed pegs in the bark. 2) "Trifoliate mild bark cracking" a symptom caused by citrus viroid IIa which induced mild to moderate bark crackingin the trifoliate trunk. 3) "Trifoliate finger imprint9'a symptom caused by citrus viroid IIIb which induced grooving in the trifoliate trunk resembling a squeezing or strangling of the trunk leaving indented imprints with horizontal striations. These three distinct symptoms in Poncirus t~ifoliata, associated with representative members of the citrus viroid groups I, I1 and I11 are described and illustrated. In addition, viroids Ia and IIIb caused significant reduction of thetrunkcross sectioninValenciascionand trifoliate rootstock, andthecanopy volume ofinfected trees.