Characterization of Erwinia amylovora Strains Isolated from Ornamentals

Fire blight, caused by the bacterium Erwinia amylovora, is a serious and destructive disease of apples (Malus spp.) and pears (Pyrus spp.) but can also infect many ornamental species in the Rosaceae family. During 2007 and 2010, typical fire blight symptoms were observed on firethorn (Pyracantha coccinea), mountain ash (Sorbus sp.), Cotoneaster horizontalis, meadowsweet (Spirea prunifolia), and crabapple (Malus floribunda). Isolated sixty three representative bacterial colonies from the ornamentals in landscape areas were characterized on the basis of biochemical, physiological, and molecular properties comparing with two reference strains of E. amylovora. Pathogenicity tests of strains were performed on the host plants. All strains induced a hypersensitive response in tobacco (Nicotiana tobaccum cv. White Burley) and produced ooze on immature pear fruits. Three primer sets, derived from PstI fragment of plasmid pEA29 and chromosomal ams region targeted, were used for PCR. The genetic diversity among the strains was investigated by random amplified polymorphic DNA (RAPD) analysis. The strains exhibited variability in susceptibility to fire blight among hosts and cultivars, and in virulence among strains of E. amylovora. In addition, all strains were identified as E. amylovora by fatty acid methyl ester profiles with similarity indices ranging from of 79% to 99%. Clustering of strains in the resulting dendrogram was not correlated with different hosts and E. amylovora strains were homogenous. Phytosanitary measures were taken to avoid spread of the pathogen to new rosaceous plants.