Early events in fire blight infection and pathogenesis of Erwinia amylovora

Abstract

When viewing the fire blight disease from the perspective of a disease cycle, it is clear that the pathogen Erwinia amylovora goes through different infection stages on an annual basis. These stages include the initial infection of flowers and shoot tips with ooze as the inoculum; systemic spread through the plant vasculature; production of ooze on plant surfaces as the secondary inoculum, and formation of annual cankers to tolerate winter stresses. Among them, the stage of initial infection of flowers and shoot tips drew most research attention, as in this stage, E. amylovora transits from epiphytic colonization on the plant surface to endophytic infection internally in the plant tissue. Limiting the epiphytic colonization of E. amylovora on flower surfaces is also the focus of fire blight management, as only the epiphytic cells of E. amylovora can be targeted by the antimicrobial sprays. In this review, we focus on some of these “early events” during the initial infection of flowers and shoot tips. We define these “early events” as any events that occur from the production of inoculum to before the systemic spread of the pathogen through the xylem. This includes the production and transfer of the inoculum (ooze) to flowers and shoot tips; epiphytic proliferation of E. amylovora on flower / leaf surfaces; entry of the pathogen cells into host through natural openings or wounds; and initial proliferation immediately after entering the host. In the first half of this review, we summarize findings of some recent pathological investigations of these early events. In the second half of this review, we discuss the pathogenesis of E. amylovora in the context of these early events, particularly how E. amylovora modulates its virulence expression to induce disease symptoms. We focus on recent findings published within the past five years.