RESULTS FROM SURVEY ON RESISTANCE OF PLUM CULTIVAR 'JOJO' TO PLUM POX VIRUS

The aim of this study was to evaluate the reaction of plum cultivar ‘Jojo’ to Plum pox virus (PPV) infection by combination of biological indexing with PPV-M, -D and -Rec strain isolates and field observations in Bulgarian commercial orchards, planted with ‘Jojo’, under the agro-biological conditions of three locations of the Plovdiv region. Development of visual symptoms on the field grown trees’ leaves and fruits was monitored during three successive vegetative cycles (2010-2012). PPV presence in plant tissues was detected by enzyme-linked immunosorbent assay (ELISA) and immune capture-reverse transcription-polymerase chain reaction (IC-RT-PCR). The biological indexing experiment showed that initially small chlorotic spots developed on the leaves of the inoculated plants that turned brownish (hypersensitive reaction) later on, followed by falling of the necrotic tissues. In the field survey, similar symptoms of hypersensitivity were observed on the leaves of some ‘Jojo’ trees. Leaf samples from plants that manifested hypersensitive reaction (the inoculated plants and field growing trees) reacted negative with antiserums against PPV in ELISA and no amplification was obtained in IC-RT-PCR with the primer pair P1/P2 used. Several of the trees grown in the field showed some virus-like leaf symptoms, during the third vegetative period. Samples collected from these trees reacted negative for PPV in ELISA and none of the samples tested by IC-RT-PCR produced PPV-specific PCR products. However, the symptomatic trees reacted positive with antiserum against Prunus necroticc ring spot virus (PNRSV). No symptoms on the fruits of the inspected ‘Jojo’ trees were recorded. The results obtained suggest that ‘Jojo’ cultivar could be resistant to PPV infection, both in the experimental biological indexing and under the agro-biological conditions of the Plovdiv region of Bulgaria.