OCCURRENCE AND IDENTIFICATION OF CITRUS VIROIDS FROM SUDAN
M.E. Mohamed, S.M. Bani Hashemian, G. Dafalla, J.M. Bové, N. Duran-Vila
The Sudanese citrus industry is mainly based on oldline cultivars of grapefruit, sweet orange and willow leaf mandarin, grafted almost exclusively on sour orange. Infectionwith graft-transmissible agents, viroids in particular, may have deleterious effects on the productivity of these citrus species and, more importantly, limits the choice of rootstocks should sour orange be replaced by alternative rootstocks because of tristeza disease. Three cv. Foster pink grapefruit sources and one local selection of Nuri-16, a local sweet orange selection, proved to be infected by several viroids, including Citrus exocortis viroid (CEVd) and Hop stunt viroid (HSVd), the causal agents of exocortis and cachexia, respectively. Cloning and sequencing of four HSVd isolates showed that in addition to the characteristic variants containing the “cachexia expression motif” or the “non-cachexia expression motif”, other variants were also present. To secure additional information, samples from 24 citrus species and cultivars were collected in 2008 from several citrus-growing regions and analyzed for the presence of viroids. The results confirmed that CEVd, HSVd, Citrus bent leaf viroid (CBLVd), Citrus dwarfing viroid (CDVd), and Citrus bark cracking viroid (CBCVd), but not Citrus viroid V (CVd-V), are widely spread throughout the surveyed areas. None of the collected samples was found to be viroid-free. The relationships between viroid infection and “gummy bark” and “Kassala disease”, two affections of unknown etiology, but with cachexia-like gum impregnations in the bark, are discussed.