Widespread occurrence of apple proliferation disease in low-intensity orchards of Basilicata

Visual symptom assessment and PCR amplification were used to survey the occurrence of apple proliferation (AP) disease in low-intensity orchards in the Agri valley, a major cultivation area of Basilicata (southern Italy). The apple trees examined, whose cultivars were not determined as they consisted mostly of local types, were more than 20-year-old. Therefore, these plants had been exposed to insect vectors for a long time. The survey revealed that a high percentage of trees were infected reaching more than 50% in some locations. The symptoms of diseased trees were generally mild and consisted of enlarged stipules, rosettes, witches’-brooms as well as subterraneous witches’-broom-like growth arising from large roots. However, the incidence and severity of symptoms in the aerial parts of affected trees were more pronounced in trees which had been heavily pruned in the previous dormant season. Specificity of the primers used and RFLP analysis of PCR-amplified 16S rDNA sequences employing SspI and BsaAI restriction endonucleases showed that the trees testing positive by PCR were infected by the AP agent ‘Candidatus Phytoplasma mali’. The high incidence of AP infections in low-intensity orchards of the Agri valley is likely due to inappropriate vector control. The trees examined were not or rarely treated with insecticides. Although a few AP-affected apple trees grown in a low-intensity orchard in the Agri valley had previously been observed, our survey shows that the distribution of AP disease in Europe extends further south than previously thought and that the climatic conditions of southern Italy are not unsuitable for this quarantine disease.