Spread and competitiveness ofPlum Pox Virus:Rec and -D strains in experimental plum orchard

Abstract

Sharka disease caused by Plum pox virus (PPV) is endemic in Bulgaria. The speed and mode of spread of PPV-Rec and PPV-D isolates, molecularly characterised in advance and introduced in an experimental plum orchard to serve as permanent sources of infection was studied from 2008 to 2015. The trees were visually inspected for symptoms development and tested by DASI-ELISA twice each year. The positive samples were serologically (-M and -D specific MAbs) and molecularly (IC-RT-PCR) strain-typed, and sequenced in (Cter)NIb-(Nter)CP coding region. Artificially inoculated trees were found infected in the spring of 2009. After 7 years 43 trees (11.2%) became naturally infected. The majority of the trees were infected by PPV-Rec (65%), while only one tree (2.5%) by PPV-D. Nevertheless PPV-M was not introduced in the orchard, this strain was found in single and mixed infections with PPV-Rec strain. None of the isolates in naturally infected trees was identical to PPV-Rec and PPV-D inoculums, while several of them were a 100% identical to the respective PPV-Rec, PPV-D or PPV-M isolates from neighbourhood-infected trees. After the initial random pattern of PPV spread, aggregation of the new infections around the trees infected in previous years was observed. The presence of only Phorodon humuli and Hyalopterus pruni complex, known as PPV vectors and their poor abundance (5.0% from all caught aphids), together with some specific features of the landscape influenced the slow natural development of PPV in the experimental orchard.