Infection rates of natural psyllid populations with ‘Candidatus Phytoplasma mali’ in South Tyrol (Northern Italy)

Apple proliferation is a severe disease of apple trees spreading in many European apple growing areas. It is caused by ‘Candidatus Phytoplasma mali’ that was shown to be transmitted through infected grafting material, via natural root grafts and by sap-sucking insects. Two psyllid species, Cacopsylla picta and C. melanoneura, that are recognised as the vectors of the disease, occur in orchards of South Tyrol (Northern Italy). The aim of this study was to assess the infection rates of natural populations of these insect species with ‘Ca. P. mali’. Two additional psyllid species (C. mali and Trioza urticae), which are frequent in some apple orchards of South Tyrol, were also investigated. A total of 801 specimens from 18 orchards was analysed using a real-time PCR procedure. While no specimen of T. urticae was found to be infected with ‘Ca. P. mali’, the mean infection rate of C. melanoneura and C. mali was below 1 %. The highest infection rate was found for C. picta, with a mean value of 11 % and peaking at 33%. Based on these results, it can be concluded that C. picta plays the major role as the vector of apple proliferation in South Tyrol. Keywords: apple proliferation, Cacopsylla mali, Cacopsylla melanoneura, Cacopsylla picta, pathogen transmission, Trioza urtica