, sp. nov., a new downy mildew species infecting the endangered Hawaiian plant var. .

var. is an endangered native Hawaiian plant, and part of the recovery plan includes repopulation using plants grown in a nursery. However, disease pressure from downy mildew is hindering repopulation efforts. The organism associated with the downy mildew was determined to be a species with brown, ellipsoid conidia measuring 21 by 16 µm on average, which was morphologically different from validly described species of that infect species, but it was morphologically similar to the invalidly published species (Art. 40.1). Comparison of mitochondrial cytochrome oxidase subunit I (1), mitochondrial cytochrome oxidase subunit II (2), nuclear internal transcribed spacer (ITS), and nuclear 28S rRNA D1-D2 (28S) loci revealed the unknown to be molecularly divergent from and , but very similar to from , the type host of . Phylogenetic trees inferred with maximum likelihood and Bayesian inference from a concatenated alignmaent and individual gene trees confirmed the divergence of the unknown from and and its similarity to . However, attempts to inoculate with the strain from var. were unsuccessful, which, in conjunction with divergence in ITS, suggests that the unknown is specific to var. . Herein, the strain on var. is described as the new species based on morphology, molecular phylogenetics, and host specificity. In addition, is described here to honor Nina Ivanova Gaponenko on the basis of her description of .