Phytophthora pachypleura sp. nov., a new species causing root rot of Aucuba japonica and other ornamentals in the United Kingdom

Isolates of an unknown Phytophthora species from the ‘ Phytophthora citricola complex’ have been found associated with mortality of Aucuba japonica in the UK . Based on morphological characteristics, growth–temperature relationships, sequences of five DNA regions and pathogenicity assays, the proposed novel species is described as Phytophthora pachypleura . Being homothallic with paragynous antheridia and semipapillate sporangia, P. pachypleura resembles other species in the ‘ P. citricola complex’ but can be discriminated by its distinctively thick‐walled oospores with an oospore wall index of 0·71. In the phylogenetic analysis based on three nuclear ( ITS , β‐tubulin, EF ‐1α ) and two mitochondrial ( cox1 , nadh1 ) DNA regions, P. pachypleura formed a distinct clade within the ‘ P. citricola complex’ with P. citricola s. str ., P. citricola E and P. acerina as its closest relatives. Phytophthora pachypleura is more aggressive to A. japonica than P. plurivora and P. multivora and has the potential to affect other ornamental species.