Diversity and phylogeny of Phytophthora Clade 9, including descriptions of three novel species

Abstract Three novel species from Phytophthora ITS Clade 9 are described from Australia and South Africa . Phylogenetic analyses were performed using data from nuclear regions (ITS, β-tubulin, heat shock protein 90 and elongation factor) and mitochondrial regions (cytochrome c oxidase subunit 1 and NADH dehydrogenase subunit 1). The three novel species share typical Clade 9 features, including non-papillate, persistent sporangia with broad, flat apices and frequent internal proliferation, and are either homothallic or sterile. Each species is nonetheless separable from related species by a distinct combination of morphological characters and temperature profiles. The molecular and morphological data supported recognition of three novel species: Phytophthora stellaris in Clade 9b , and P. brevifila and P. calida in the newly proposed Clade 9e. Surveys in Australia and South Africa also revealed additional but as yet undescribed taxa, including several single-isolate lineages and a complex of polymorphic isolates closely related to P. honggalleglyana and P. virginiana , likely of hybrid origin. These findings expand the known diversity, morphology and biogeography of Clade 9. Although few Clade 9 species are currently associated with serious plant disease, documenting their diversity is critical for establishing baselines, clarifying species placement and informing future monitoring. By formally describing novel species and recognising unresolved complexes, this study provides an essential reference point for researchers navigating one of the most taxonomically complex clades of Phytophthora .