Survival and Possible Spread of Erwinia amylovora and Related Plant-Pathogenic Bacteria Exposed to Environmental Stress Conditions

The fire blight pathogen Erwinia amylovora was assayed for survival under unfavourable conditions such as on nitrocellulose filters, in non-host plants as well as in inoculated mature apples and in infested apple stem sections. In a sterile dry environment, an E. amylovora EPS (exopolysaccharide) mutant, and to a lesser extent its parental wild-type strain decreased within 3 weeks to a low titre. However, under moist conditions the decrease of viable cells occurred only partially for both strains. Very low cell titres were recovered after application of E. amylovora onto the surface of tobacco leaves, whereas infiltration into the leaves produced lesions (hypersensitive response, HR), in which the bacteria survived in significant amounts. A similar effect was found for the necrotic zones of HR in tobacco leaves caused by E. pyrifoliae, by Pseudomonas syringae pathovars and HR-deficient E. amylovora mutants or mutants deficient in EPS synthesis and disease-specific genes. During 7 years of storage, the viability of E. amylovora in wood sections from fire blight-infested apple trees declined to a low titre. In tissue of mature apples, E. amylovora cells slowly dispersed and could still be recovered after several weeks of storage at room temperature. A minimal risk of accidental dissemination of E. amylovora apart from infested host plants can experimentally not be excluded, but other data confirm a very low incidence of any long distance distribution