A new oomycete pathogen <i>Olpidiopsis dasysiphoniae</i> sp. nov. (Oomycota) infecting the red alga <i>Dasysiphonia japonica</i> (Ceramiales, Delesseriaceae)

Abstract Olpidiopsis is one of the major pathogens of commercially valuable red algae. Despite being such an important pathogen, the infection process has been studied and documented mainly in the Olpidiopsis species infecting Pyropia spp., but less so in other red algae. We collected an Olpidiopsis -like pathogen infecting Dasysiphonia japonica in Japan, and it is different from any previous described Olpidiopsis species. Molecular phylogeny inferred from 18S rRNA gene, cox 1, and cox 2 sequences and species delimitation analyses (i.e., Assemble Species by Automatic Partitioning, single-threshold General Mixed Yule Coalescent, Bayesian Poisson Tree Processes) showed that the collected Olpidiopsis sp. formed a supported clade with other Rhodophyta-infecting Olpidiopsis species, and was considered a distinct species by all species-delimitation methods. Furthermore, we tested the host range of this pathogen and studied its morphology through microscopic analysis. This new pathogen was distinguished from other species by the presence of only one sporangium per host cell and, during development, the sporangium became irregularly shaped, almost filling the entire host cell and produced up to 14 discharge tubes to release zoospores. The host range tests found that it only infected species of Dasysiphonia spp. Therefore, we propose the name, Olpidiopsis dasysiphoniae sp. nov., for this new species.