Diversity and Pathogenicity ofPhytophthoraSpecies Associated with Declining Alder Trees in Italy and Description ofPhytophthora alpinasp. nov

Extensive decline and mortality events of alder trees have recently been observed in several riparian ecosystems in Italy. Since there is little information about the aetiology of this disease and given the high ecological relevance of riparian ecosystems, an in-depth study was conducted in three sites spanning from the Mediterranean to Alpine regions. From spring 2019 to spring 2020, 261 samples of bleeding cankers, rhizosphere soil and leaves used as baits along waterways were collected and used forPhytophthoraisolation. Based on morphology, colony appearance and DNA sequence data, 10 species belonging to 6 clades were identified. These includedP. plurivora(84 isolates),P. pseudocryptogea(50),P. hydropathica(18),P. gonapodyides(14),P. bilorbang(13),P. pseudosyringae(12),P. lacustris(11),P. acerina(7),P. cactorum(1) and one isolate of the hybridPhytophthora xserendipita. In addition, two newPhytophthoraspecies, one of which is described here asPhytophthora alpinasp. nov., were isolated. The pathogenicity ofP. alpinaand other species obtained from samples collected in the green alder stand was assessed on 3-year-old seedlings. All species proved to be pathogenic on green alder causing symptoms congruent with field observations. Results obtained have allowed us to expand knowledge about alder decline aetiology. The diversity of pathogenicity ofPhytophthoraspecies associated with symptomatic alder trees suggested that no single agent is responsible for the disease, but that it is the result of multiple infections of differentPhytophthoraspecies, variable in assemblages among sites.