Occurrence, epidemiological aspects and multilocus sequence typing of pear decline phytoplasma in southern Italy

Visual symptoms assessment and PCR assays using rDNA-directed primers, followed by RFLP analysis, were employed to update and extend knowledge on the occurrence of pear decline (PD) phytoplasma in several pear-growing areas of the neighboring regions Campania, Basilicata and Calabria of southern Italy, including those which had previously not been examined. The PD phytoplasma was detected in all symptomatic pear trees examined and in 46 out 63 pools of the pear psylla Cacopsylla pyri insects captured in the surveyed pear orchards. Disease incidence of PD ranged from 10 to 65% in the pear orchards examined. Severe PD symptoms were observed in symptomatic pear trees grafted onto BA29 quince rootstock. Multilocus sequence typing (MLST) analysis of aceF, pnp, imp and secY gene sequences revealed a considerable genetic variability among 30 selected PD phytoplasma-infected samples examined in southern Italy. The highest number of genotypes was observed for pnp and secY genes (five genotypes for each), whereas the highest sequence dissimilarity value between the samples under study 15.1 occurred in the aceF gene. For each gene examined, the most frequently identified genotypes were A24, P4, I40 and S13, respectively. The detected multilocus profiles are compatible with the hypothesis of possible interspecies recombination between ‘Ca. Phytoplasma pyri’ and ‘Ca. Phytoplasma prunorum’, although alternative explanations including mixed infections, preferential amplification and PCR-mediated recombination are not excluded. Fifteen distinct multilocus profiles were obtained from the selected samples and compared with previously published profiles using a similarity network. These findings extend knowledge on the multilocus diversity of PD phytoplasma in southern Italy.