High diversity of <i>Phytophthora</i> species in natural ecosystems and nurseries of Portugal: Detrimental side effect of plant introductions from the age of discovery to modern globalization

Abstract Between 2010 and 2015, an extensive survey of Phytophthora diversity was performed across Portugal. Baiting and direct isolation tests obtained 48 Phytophthora taxa in 62 of 70 forest stands (89%) from 148 of 302 rhizosphere soil samples (49%) and 25 of 29 tree species (86%), and from bark cankers of Alnus lusitanica and Fagus sylvatica ; from 33 of 34 rivers and forest streams (97%); and in 12 of 13 forest nurseries (92%) from 54 of 87 root ball samples (61%) and 16 of 21 tree species (76%). The 799 Phytophthora isolates belonged to 28 known and three recently described species; one designated and two new non‐hybrid taxa; and five designated and nine new hybrid taxa. The majority, 34 Phytophthora taxa (71%), are introduced aliens in Europe while nine (19%) and five (10%) taxa are of cryptic and native origin, respectively. Phytophthora cinnamomi was the most common species in forests (56%) and forest nurseries (61%). Multiple first records for Portugal and partly Europe of both Phytophthora taxa and tree declines and new host– Phytophthora associations were found. Isolation rates from forest soil differed considerably between declining (65%) and healthy trees (10%), regardless of the tree species. The widespread infestations of nurseries with aggressive wide‐host range pathogens like P . cinnamomi , P . pseudocryptogea , P . × cambivora , P . multivora and P . plurivora , and host‐specific pathogens like P . alticola , P . quercetorum and P . quercina , will inevitably cause their further spread to the wider environment, exacerbating pathogen threats to forest ecosystems and thus weakening their resilience to climatic extremes.