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.

中文翻译：

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火疫病感染的早期事件和梨火疫病的发病机制

从病害周期的角度来看火疫病，很明显，梨火疫病菌每年都会经历不同的感染阶段。这些阶段包括以软泥为接种物的花朵和芽尖的初始感染；通过植物脉管系统系统性传播；在植物表面产生软泥作为二次接种物，并形成一年生溃疡病以耐受冬季压力。其中，花和芽尖的初始侵染阶段最受关注，在这个阶段，梨火疫病菌从植物表面的附生定植过渡到植物组织内部的内生侵染。限制梨火疫病菌在花表面的附生定植也是火疫病管理的重点，因为只有梨火疫病菌的附生细胞。淀粉样病毒可以成为抗菌喷雾的目标。在这篇综述中，我们重点关注花朵和枝条初始感染期间的一些“早期事件”。我们将这些“早期事件”定义为从接种物产生到病原体通过木质部系统传播之前发生的任何事件。这包括接种物（软泥）的生产和转移到花和芽尖；E. amylovora 在花/叶表面的附生增殖；病原体细胞通过自然开口或伤口进入宿主；并在进入宿主后立即开始增殖。在这篇综述的前半部分，我们总结了最近对这些早期事件的一些病理学调查的结果。在本综述的后半部分，我们在这些早期事件的背景下讨论了梨火疫病菌的发病机制，尤其是梨火疫病菌如何调节其毒力表达以诱发疾病症状。我们专注于过去五年内发表的最新研究结果。