An Emerging Association of Bud Union Crease and Pararetrovirus on Citrus in Italy

Field surveys conducted in citrus groves in Sicily, southern Italy, revealed sweet orange trees of different cultivars (‘Moro’, ‘Navelina’, and ‘Newhall’) grafted on ‘Forner Alcaide 5’ and citrange ‘C35’ showing typical symptoms of bud union crease in the canopy and at the grafting point. In order to better understand the possible etiology of this disorder, symptomatic samples were tested for the most common citrus viruses and viroids. Moreover, the identity of the surveyed rootstocks was confirmed with molecular analyses. In addition, to investigate the possible presence of viral and viroid agents not detected by targeted molecular assays, all citrus samples were subjected to high-throughput sequencing (HTS) analysis. Results showed that all samples resulted negative to all the tested viruses and viroids. Only one sample tested positive to Hop stunt viroid. Results from high-throughput sequencing generated sequence reads corresponding to Citrus Blight-associated Pararetrovirus (CBaPRV), representing the predominant “pathogen”-related sequences detected in the HTS dataset. The found association does not prove the causative effect with the bud union crease but highlights an important result that needs further investigations.