Pseudomonas cerasi, the new wild cherry pathogen in Serbia and the potential use of recG helicase in bacterial identification

In May 2016, an unusual appearance of leaf spot (water-soaked, brown-purple, round to angular surrounded with yellow halos) was observed on the leaves of wild cherry specimens grown in Rimski Sancevi, Vojvodina (North Serbia). The causal pathogen was isolated from the wild cherry diseased leaves on Nutrient Agar supplemented with 5% sucrose and identified as Pseudomonas cerasi based on multilocus sequence analysis (MLSA). PCR amplification and sequencing of four housekeeping genes-gapA, gltA, rpoD and gyrB-showed 100% (gapA, gltA), 99.81% (rpoD) and 99.67% (gyrB) identity with P. cerasi type strain CFBP8305(T) (=58(T) = LMG28609(T)) and strain PL963 sequences from NCBI database. Pseudomonas cerasi isolates (coded as RE10-RE19) were LOPAT +- - - + (Pseudomonas Group Ia) and GATTa + - + -, produced fluorescent pigment, were able to utilise lactic and aspartic acid, but not tartaric acid. All isolates were pathogenic on wild cherry seedlings and leaves, immature sweet and sour cherry fruitlets, as well as on lilac leaves and green bean pods. This study also aimed to design new primers (recG-F/recG-R) for amplification of recG gene (encoding ATP-dependent DNA helicase RecG) known for its crucial role in DNA recombination and repair. Amplification of this gene enabled high identification ability of our wild cherry isolates as well as isolates belonging to the closely related stone fruit pathogenic Pseudomonas species. The obtained results highlight the potential of using recG gene for the specific detection and the identification of pathogenic Pseudomonas syringae complex. This research presents the first report of P. cerasi infecting wild cherry as well as first description of this bacterium in Serbia. Obtained results indicate the risk from further spread of this bacterium with the infected cherries rootstock, having a key role in its epidemiology.