Pear decline: the fate of phytoplasmas in an Italian orchard

Introduction Pear decline (PD) is a phytoplasma-associated disease that caused severe losses in production since the fifties of the last millennium in the main pear producing areas in both Europe and USA. While in the latter areas the pear production was dramatically reduced, in Italy pear production is still active although not increasing, however, together with fire blight the PD is still one of the most spread diseases in orchards. Previous reports indicate a ‘Candidatus Phytoplasma pyri’ movement in the infected trees from roots to upper parts according to seasonality (1). To verify such hypothesis a four-year field trial was set in a severely PD affected orchard located in Modena province (Italy). Materials and Methods In a four pear row (120 five year-old plants each) of cv Abate Fétel grafted on quince EMH (QR 193-16) plants were selected in September 2013 and tested by nested PCR/RFLP analyses on 16S rDNA (2) for PD presence. A total of 8 symptomatic and PD positive plants and 4 asymptomatic and PD negative plants as control was then individually covered by insect-proof nets in February 2014 after a deltamethrin treatment. Symptom monitoring was carried out in the plot in each of the subsequent years together with phytoplasma presence PCR/RFLP analyses in both leaves and roots of the covered selected plants. Insect presence in the plot and inside the single covered plants was monitored by yellow sticky traps. Results and Discussion The monitoring of the plants under insect-proof nets showed that, while the asymptomatic plants remained negative, those showing symptoms in 2013 had a reduced presence of symptoms over the years and in 2016 resulted all negative for PD presence, suggesting that no seasonal translocation of ‘Ca. P. pyri’ was present in the absence of the insect vector. The presence of Cacopsylla pyri, C. pyricola and C. pyrisuga was registered in the remaining of the plot where the uncovered plants showed symptoms in scattered plants that resulted PD positive, confirming the phytoplasma spreading in relationship with insect vectors presence. Since 2015 in this and other orchards of the area there is a strong infestation of Halyomorpha halys that was reported as aster yellows phytoplasma vector in China and recently confirmed under laboratory conditions (3). Work is now in progress to verify if it can also play a role in the spreading of this phytoplasma disease.