Citrus dwarfing viroid: effects on tree size and scion performance specific to Poncirus trifoliata rootstock for high-density planting

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-year-old 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 x 6.7 m) or high-density (3 m x 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.