Diversity and distribution of Phytophthora species across different types of riparian vegetation in Italy with the description of Phytophthora heteromorpha sp. nov

Riparian formations encompass a diverse suite of transitional zones between terrestrial and aquatic ecosystems. During the last decades, these formations have been impacted by several emerging diseases. The first outbreaks were detected on alder formations, but have progressively also been observed on other plant species such as , , , , and . Declining plants showed a plethora of symptoms (leaf spot, shoot blight, bleeding cankers and root rot) indicative of infections. Since there is little information about the aetiology of these pathosystems, from November 2019 to March 2023, an in-depth study was conducted in 46 riparian ecosystems spanning from the Mediterranean to Alpine regions. Overall, 744 symptomatic samples (stem bleeding cankers and root with rhizosphere) from 27 host species were collected for isolation. Based on morphology and DNA sequence data, 20 known species belonging to seven phylogenetic clades have been identified: (202 isolates), (156), (84), (57), (31), (30), (20), (19), (13), (13), (9), (6), (6), (4), (4), (3), (2) (2), (2) and (2). In addition, 26 isolates of a new putative species obtained from and are described here as sp. nov. The new species proved to be pathogenic on grey alder causing symptoms congruent with field observations. This study represents the most comprehensive investigation on the species associated with declining riparian vegetation in Italy and highlights that the polyphagous pathogen represents a growing threat to Mediterranean, temperate and alpine ecosystems.