Xylella fastidiosa (Xf) is classified as a quarantine pest due to its consequences on economically significant crops. Its main form of transmission in Europe is through the insect Philaenus spumarius. Due to climate change, the populations of insect vectors have become more extensive, resulting in the dissemination of the bacteria over longer periods, but the destruction of these insects raises issues due to their role in nature. Upon infection, Xf causes the occlusion of xylem vessels via bacterial aggregates and tylosis production by the plant as a response to infection. Although symptomatic manifestations of Xf are often linked to water stress, a variety of plant species have been found to carry the pathogen without symptoms, making it all too easy to evade detection when relying on visual inspections. Beyond water stress, other conditions (individual plant resistance/tolerance, bacterial concentrations, transpiration rates, and interactions between subspecies) may be implicated in symptom development. A thorough understanding of how this disease develops, especially its capacity to spread from the initial focus and establish a systemic infection, is imperative. This review focuses on the Xf infection process, the development of symptoms, its spread within Portugal, and the actions that have been taken to counter it.
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