Symptoms expression and growth of citrus varieties on citrange rootstocks inoculated with different Citrus Viroids.

The Citrus tristeza virus (CTV) epidemics in Italy require the substitution of sour orange, a rootstock highly susceptible to CTV isolates but tolerant to Citrus viroids (CVds), with citranges that on the contrary result tolerant to CTV. Among the CVds just two are responsible of citrus diseases: Citrus exocortis viroid (CEVd) for exocortis and Hop stunt viroid (HSVd) for cachexia respectively. CEVd causes bark scaling of trifoliate orange and its hybrids, whereas its performance on citranges and other hybrids has been questioned several times and it’s still unclear. The behaviour of CEVd and/or different citrus viroids was evaluated on Tarocco sweet orange and Clementine comune grafted on Troyer and Carrizo citranges, respectively inoculated 28 and 15 years ago. Trees were monitored every year by field inspections for symptom observations, assays on Etrog citron, RT-PCR and real time RT-PCR analysis. CEVd-inoculated Clementine trees (2x3 m density planting) showed no bark scaling but only a size reduction, whereas those inoculated with a mixture of Citrus dwarfing viroid (CDVd), HSVd and CEVd evidenced reduction in size, bud union line indentation and bark gumming. Tarocco sweet orange performed very well in size and yield despite CEVd infection likewise healthy trees.