Are D and Rec strains of Plum pox virus similar or different in terms of competitiveness and symptomatology?

Plum pox virus (PPV) is considered the most detrimental viral pathogen of stone fruits, causing important economic losses. Exhaustive studies revealed that D, M and Rec are the prevalent strains of PPV in Europe. While different experimental reports revealed that PPV-M is more aggressive than PPV-D, limited information is available about the competitiveness of PPV-Rec under field conditions. Also, symptoms developed by PPV-Rec infection are scarcely documented. To increase this knowledge, we selected and monitored an experimental plum orchard already contaminated by both PPV-D and PPV-Rec. The disease spread was monitored by visual observation and ELISA testing each year along five consecutive vegetative periods. The dynamic of single and mixed infections was assessed by molecular strain typing (IC-RT-PCR) of all newly and single-infected trees. While the disease prevalence increased almost linearly, the number of single PPV infected trees (either by PPV-Rec or by PPV-D) remained relatively stable during the five-year period of monitoring, which is explainable by the continual increase of the number of trees infected with both strains together. No significant difference in the progression rate between D and Rec strains was found when considering their presence in both single and mixed infections. Although the types of PPV symptoms developed on leaves and fruits varied among cultivars, no clear-cut D or Rec strain-specific symptoms were developed within the same cultivar. Overall results revealed that PPV-D and PPV-Rec strains had similar behaviour in terms of competitiveness and symptoms developed under our experimental field conditions.