Ecological impact, epidemiology and genetic diversity of invasive Phytophthora species in mountain ecosystems

The progressive introduction of alien and invasive pathogens in natural ecosystems represents one of the main threats to forest health worldwide. Among the main destructive plant pathogens accidentally introduced to new areas every year by the trade in plants and goods, deforestation and reforestation activity, change in land use, irrigation systems, urbanization and recreative forest activity, many belong to the genus Phytophthora. This oomycete genus currently encompasses over 220 species grouped into 12 well-defined phylogenetic clades. The global biodiversity of Phytophthora species has been extensively investigated over the last decades with the description of about a hundred new species. Notably, the development of molecular tools phylogenetic analysis has revealed an unexpected diversity of species in both natural and agricultural ecosystems in temperate and tropical areas. Despite the intense research activity on this group of pathogens worldwide, the presence and impact of Phytophthora species in mountain ecosystems remain underreported leaving a significant gap in our knowledge. Therefore, the main goal of this PhD project was to explore the diversity of Phytophthora species across different types of mountain and alpine forests in Europe in order to fill the gaps on the occurrence, distribution and ecological impact of these emerging pathogens on mountain vegetation. Field surveys conducted in different Italian, Portuguese and Slovenian forests over a three-year period showed a complex of 26 pathogenic Phytophthora species associated with leaf and shoot blights, bleeding cankers and root rot symptoms on 41 plant species typical of mountain vegetation. The variety of symptoms observed on plants were compatible with both air and soil-borne Phytophthora infections. The extensive field surveys conducted in this study highlight that severe disease outbreaks and mortality events are currently affecting shrubs and trees especially along riparian habitats. Phytophthora pseudosyringae and P. plurivora were the dominant species in the monitored sites, in particular, P. pseudosyringae was prevalent in the cold environments. Overall the species from clades 2 and 6 were the most common; they were recovered from different types of vegetation, streams and riparian ecosystems. Phytophthora communities found in the mountain vegetation show an unexpected highest diversity, including two new species namely P. pseudogregata and P. heteromorpha. A further new species, P. mediterranea, was discovered from myrtle plants grown in nurseries, this highlights the increasing risk posed by nursery material to natural ecosystems. The riparian habitats were identified as the major natural corridors correlated with the diffusion of Phytophthora species in mountain ecosystems. In conclusion, this study contributes to expand knowledge on the ecology of Phytophthora species in both natural areas and nurseries with 87 new host-pathogens associations and 10 new Phytophthora reports for Italy, 11 for Portugal and 6 for Slovenia.