Six new Phytophthora species from ITS Clade 7a including two sexually functional heterothallic hybrid species detected in natural ecosystems in Taiwan

During a survey of Phytophthora diversity in natural ecosystems in Taiwan six new species were detected. Multigene phylogeny based on the nuclear ITS, ß-tubulin and HSP90 and the mitochondrial cox1 and NADH1 gene sequences demonstrated that they belong to ITS Clade 7a with P. europaea , P. uniformis , P. rubi and P. cambivora being their closest relatives. All six new species differed from each other and from related species by a unique combination of morphological characters, the breeding system, cardinal temperatures and growth rates. Four homothallic species, P. attenuata , P. flexuosa , P. formosa and P. intricata , were isolated from rhizosphere soil of healthy forests of Fagus hayatae , Quercus glandulifera , Q. tarokoensis , Castanopsis carlesii , Chamaecyparis formosensis and Araucaria cunninghamii . Two heterothallic species, P. x heterohybrida and P. x incrassata , were exclusively detected in three forest streams. All P. x incrassata isolates belonged to the A2 mating type while isolates of P. x heterohybrida represented both mating types with oospore abortion rates according to Mendelian ratios (4–33 %). Multiple heterozygous positions in their ITS, ß-tubulin and HSP90 gene sequences indicate that P. x heterohybrida , P. x incrassata and P. cambivora are interspecific hybrids. Consequently, P. cambivora is redescribed as P. x cambivora without nomenclatural act. Pathogenicity trials on seedlings of Castanea sativa , Fagus sylvatica and Q. suber indicate that all six new species might pose a potential threat to European forests.