Peculiar plum pox potyvirus D-populations are epidemic in peach trees

Two hundred field isolates, collected in the main foci of infection detected in southern France have been characterized using electroblot-immuno-assay and in few cases, RFLP analysis. The occurrence in France of two main populations (90% of the isolates) showing distinct electrophoretic mobility of their coat protein is confirmed. These PPV populations have distinct prevalence and show uneven distribution in peach and apricot orchards, reflecting a difference in their epidemic potentialities. However, the detection of isolates showing a PPV-D polymorphism (but mostly with atypical electrophoretic mobility) able to induce epidemics in peach orchards constitute a new but still unfrequent epidemiological situation. Several isolates have been more precisely characterized by direct sequencing of amplified PCR fragments located within the 3' and 5' ends of coat protein gene. A phylogenetic analysis including published nucleotide sequences of several European isolates has shown that the French PPV populations fall into the two main PPV-D and PPV-M of the four molecular groups previously determined among PPV populations. However, a French PPV-D isolate coming from peach reveals several amino acid substitutions in the N-terminal region of the coat protein which have never been found in all PPV isolates sequenced so far. Several biological experiments have been undertaken in order to evaluate epidemic potentialities of these isolates. It appears that efficiency to infect young peach trees after aphid inoculation, only observed under insect-proof conditions, is a selective factor and could explain the adaptation to peach of particular PPV-D isolates.