Citrus Viroids: Symptom Expression and Performance of Washington Navel Sweet Orange Trees Grafted on Carrizo Citrange.

Citrus are natural hosts of several viroid species. Citrus exocortis viroid (CEVd) and Hop stunt viroid (HSVd) are the causal agents of two well-known diseases of citrus, exocortis and cachexia. Other viroids have been found to induce specific symptoms and different degrees of stunting in trees grafted on trifoliate orange and trifoliate orange hybrids. A field assay was initiated in 1989 to establish the effect of CEVd, HSVd, Citrus bent leaf viroid (CBLVd), Citrus dwarfing viroid (CDVd), and Citrus bark cracking viroid (CBCVd) on Washington navel sweet orange trees grafted on Carrizo citrange rootstock. Here we report the effect of viroid infection on symptom expression, tree size, fruit production and quality evaluated from 2004 to 2007. Vegetative growth was affected by viroid infection with height and canopy volume being reduced. No bark scaling symptoms were observed in CEVd-infected trees albeit they presented lesions and blisters in the roots. Bark cracking symptoms were consistently observed in CBCVd-infected trees that were smaller with enhanced productivity and fruit size. No major effects were found as a result of infection with CBLVd, HSVd, or CDVd. The quality of the fruits was not affected by viroid infection, except for the low diameter of the fruits harvested from HSVd-infected trees. An interesting effect was identified in terms of tree productivity increase (yield/canopy volume) as a result of infection with CEVd, CDVd, and especially CBCVd.

Effect of a Field-Source Mixture of Citrus Viroids on the Performance of 'Nules' Clementine and 'Navelina' Sweet Orange Trees Grafted on Carrizo Citrange.

A field-source mixture of citrus viroids was characterized and shown to contain Citrus exocortis viroid (CEVd), Hop stunt viroid (HSVd), Citrus bent leaf viroid (CBLVd), and Citrus dwarfing viroid (CDVd). Sequencing results showed that: (i) CEVd contained the PL and PR characteristic of class A variants; (ii) HSVd was a noncachexia variant; (iii) CBLVd was related to CVd-Ia variants; (iv) CDVd was a mixture of two types (CVd-IIIa and CVd-IIIb) of variants. The presence of the same type of variants in inoculated clementine (Citrus clementina 'Nules') and sweet orange (C. sinensis 'Navelina') trees on Carrizo citrange (Poncirus trifoliata × C. sinensis) rootstocks was confirmed. The effect of infection was determined by assessing the performance of infected and noninfected trees growing in the field. Infection resulted in small trees with reduced canopy, yielding a reduced crop. Fruit characteristics were also affected: (i) clementine and sweet orange fruits from infected trees were larger than those from noninfected trees; (ii) clementine fruits from infected trees differed in shape from those of noninfected trees; (iii) sweet orange fruits from infected trees had maturity indexes and juice contents higher than those from noninfected trees; (iv) in both species, the density of the juice, the amount of soluble solids, and the acidity of the fruits from infected trees were lower than those of fruits from noninfected trees. Infected trees had a poorly developed root system with fibrous roots containing fewer amyloplasts than noninfected trees. The results of an in vitro assay on the induction and development of roots in cultured explants are discussed.