Bacterial canker pathogens present in the materials of Prunus armeniaca propagation

The aim of this study was to find out whether causal agents of bacterial canker and the premature death of apricot trees are present in the internal tissues of propagating material of various origins. In total, 33 samples of apricot ( Prunus armeniaca L.) scion materials from eight and six samples of rootstocks from four European localities were analysed for the presence of the pathogenic Pseudomonas species. Significant differences were found in the diversity of the culturable bacterial population and the incidence of Pseudomonas bacteria in internal tissues of scion and rootstock buds. In most of the rootstocks, no Pseudomonas bacteria were detected. A total of 148 Pseudomonas -like strains isolated from scion samples were collected mainly from localities F1–F5 (in France) and G1–G2 (in Germany). These strains were clustered into 11 groups by means of fatty acid methyl ester (FAME) analysis; 91.9% of them were able to induce a hypersensitive reaction on tobacco leaves and 37.8% were positive in ice nucleation activity. A set of 89 Pseudomonas strains was characterised by means of Psy -PCR targeting of the Pseudomonas syringae ( Ps ) complex, complex phylogroup (PG), syrB -PCR targeting of the syringomycin synthesis, pathogenicity on detached apricot twigs, and the ability to survive within epiphytic microflora. Pseudomonas strains attributed to PG01, PG04, and PG07–08 were slightly pathogenic to detached apricot twigs. Altogether, 5% of Pseudomonas strains highly pathogenic to detached apricot twigs were isolated from apricot scion samples originated in localities F2, F5, and G1. These strains were attributed to PG02 and PG03, and rpoD sequencing confirmed a similarity to strains of P. syringae pv. syringae and P. amygdali pv. morsprunorum known to be pathogenic to apricot, respectively. The occurrence of these pathogens in buds of scion materials increases the risk of bacterial canker and the premature death of young apricot trees.