Citrus psorosis virus Bark Scaling on Tarocco Sweet Orange

Despite the wide distribution of vein flecking of citrus leaves in Italy, psorosis bark scaling has been reported only on Navelina sweet orange (Citrus sinensis L.), and Thompson and Washington navel oranges (1). Infection has not been found on any local sweet orange cultivars. Among these is Tarocco, a sweet orange cultivar that originated in Sicily, is very common because of the high fruit quality, has an attractive fruit appearance, and has blood-red pigmented flesh. In April 2001, classic psorosis bark scaling symptoms were observed on the main limbs of 10-year-old Tarocco trees grafted on sour orange rootstock, originally obtained by topworking Clementine (C. reticulata Blanco) trees with scions collected from 19-year-old Tarocco trees with no bark scaling at that time. The symptomatic trees first displayed one or two isolated circular patches of scales with gumming on the main or secondary limbs. As the disease progressed, the number of patches increased and coalesced to form bigger scales, resulting in bark flaking. Approximately 15% of trees in the field showed different stages of the disease. All of the affected trees showed vein flecking of young leaves. A leaf pattern was also present in a few plants without bark symptoms. Bark symptoms were correlated with the presence of Citrus psorosis virus (CPsV). Samples (110 representing 10% of the total number of trees in a field located in the area of Catania, Sicily) were collected during the spring flush using a W-pattern sampling method and tested by double-antibody sandwich enzyme-linked immunoassay (DAS-ELISA) (monoclonal antibody [MAb] PS29) (2). Of trees tested, 14% showed bark scaling, and 86% were symptomless. All symptomatic plants were tested and 70% of symptomless trees were positive based on DAS-ELISA. To confirm DAS-ELISA results, 10 field samples were also tested by bioassay on indicator plants (Navelina sweet orange ISA 315 and Pineapple sweet orange), triple-antibody sandwich enzyme-linked immunoassay (TAS-ELISA), and immunosorbent electron microscopy (ISEM) with a different antiserum (MAb 13C5). DAS-ELISA-positive samples produced vein flecking on indicator plants, were positive based on TAS-ELISA, and contained typical CPsV particles based on ISEM (R. G. Milne, IFA, CNR, Turin). To our knowledge, this is the first demonstration of psorosis bark scaling reaction of Tarocco sweet orange due to CPsV infection. References: (1) A. Catara et al. Proc. Int. Soc. Citri. 1:426, 1981. (2) M. Tessitori et al. Proc. 15th Conf. IOCV, IOCV, Riverside. In press.