Characterisation of Pseudomonas syringae isolates from apricot orchards in north-eastern Italy

Abstract

A set of 31 isolates were obtained from apricot (Prunus armeniaca L.) orchards showing symptoms of blast and gummosis in orchards located in north-eastern Italy, mostly during three subsequent growing seasons (2014–2016). These isolates were identified by means of their colony morphology, pathogenicity (hypersensitive reaction in tobacco leaves and the detection of both syringomycin and coronatine coding genes), genetic features (BLASTn analysis of the rpoD gene) and physiological tests (GATTa) as Pseudomonas syringae pv. syringae and P. syringae pv. morsprunorum race 1 and race 2 strains, respectively. Phylogenetic analysis based on the rpoD gene sequence showed that the two pathovars and two races are well separated and highlighted high intrapathovar/race variation for both P. syringae pv. syringae and P. syringae pv. morsprunorum race 2. Pseudomonas syringae pv. morsprunorum race 1 formed a unique clade that had high sequence homology. Repetitive element-based polymerase chain reaction fingerprinting analysis using the ERIC, REP and BOX primers again showed both high intra-pathovar variation within the P. syringae pv. syringae isolates, which grouped into six distinct clusters, and notable intra-race genetic diversity within both P. syringae pv. morsprunorum races. All 31 strains showed tolerance to copper at 200 ppm in vitro. Among these isolates, 13 were highly resistant (up to 500 ppm) and revealed the presence of the cusCBA gene cluster. All P. syringae pv. syringae strains showed ice nucleation activity, and nine were markedly active (up −3 °C) and characterised by the presence of the INAz gene. Finally, this study is the first report of the isolation of P. syringae pv. morsprunorum race 2 strains from apricot in Italy.