| Summary: | New and emerging <i>Phytophthora</i>-related diseases in small trees, shrubs and herbaceous plants typical of subalpine vegetation have recently been observed in Italy and Slovenia. Diseased plants showed a complex symptomatology including foliar necrosis, fruit rot, shoot blight and branch bleeding cankers. Since little information is available about the aetiology of these aerial <i>Phytophthora</i> diseases, from 2019 to 2022, field surveys were conducted in 54 sites to define the occurrence, distribution and impact of the <i>Phytophthora</i> species on mountain vegetation. A total of 360 <i>Phytophthora</i> isolates were obtained from 397 samples collected from 33 herbaceous and woody host species. Based on phylogenetic analysis and morphometric data, 17 <i>Phytophthora</i> species were identified: <i>P. pseudosyringae</i> (201 isolates), <i>P. plurivora</i> (54), <i>P. gonapodyides</i> (21), <i>P. ilicis</i> (20), <i>P. alpina</i> (17), <i>P. acerina</i> (11), <i>P. cactorum</i> (7), <i>P. pseudocryptogea</i> (6), <i>P. cambivora</i> (5), <i>P. idaei</i> (4), <i>P. psychrophila</i> (3), <i>P. bilorbang</i> (2), <i>P. chlamydospora</i> (2), <i>P. hedraiandra</i> (1), <i>P. kelmanii</i> (1), <i>P. rosacearum</i> (1) and <i>P. syringae</i> (1). In addition, three isolates of a new putative <i>Phytophthora</i> species obtained from <i>Alnus viridis</i>, <i>Juniperus communis</i> and <i>Rhododendron ferrugineum</i> are described here as <i>Phytophthora pseudogregata</i> sp. nov. Overall, the results highlighted an unexpectedly high diversity of <i>Phytophthora</i> species in mountain areas, with many species able to cause aerial infections due to the production of caducous sporangia.
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