Invasive forest pathogens are a major threat to forests worldwide, causing increasing damage. The knowledge of both the specific traits underlying the capacity of a pathogen to become invasive, and the attributes predisposing an environment to invasion are to be thoroughly understood in order to deal with forest invasions. This paper summarizes the historical knowledge on this subject. Many aspects of the ecological processes underlying alien forest pathogens invasions are still unknown, which raises several scientific issues that need further study. The introduction of invasive forest pathogens to areas where naïve hosts are found, is mainly due to global plant trade. Rapid transportation and reduced delivery times increase the chances of survival of pathogen propagules and of their successful establishment in new environments. In forest pathogens, the reproduction mode seems not to be a crucial determinant of invasiveness, as highly destructive pathogens have a variety of reproductive strategies. The most important drivers of forest pathogen invasions appear to be (a) great adaptability to new environmental conditions; (b) efficient dispersal over long and short distances, possibly assisted by the capacity to form novel associations with endemic and/or alien insect vectors; (c) the ability to exchange genetic material or hybridize with resident or alien species. Moreover, these features interact with some key traits of the invaded environment, e.g. environmental variability and biodiversity richness. Host resistance and natural enemies may occur as a result of rapid selection/adaptation after the epidemic phase of invasion.
