Peronospora kuewa sp. nov., a new downy mildew species infecting the endangered Hawaiian plant Plantago princeps var. princeps

Authors: DAVIS, WILLIAM - Orise Fellow; KO, MANN - Hawaii Department Of Agriculture; OCENAR, JODIE - Hawaii Department Of Agriculture; ROMBERG, MEGAN - Animal And Plant Health Inspection Service (APHIS); THINES, MARCO - Goethe University; Crouch, Jo Anne

Submitted to: Mycologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/4/2021
Publication Date: 3/18/2021
Citation: Davis, W.J., Ko, M., Ocenar, J.R., Romberg, M., Thines, M., Crouch, J.A. 2021. Peronospora kuewa sp. nov., a new downy mildew species infecting the endangered Hawaiian plant Plantago princeps var. princeps. Mycologia. 113(3):643-652. https://doi.org/10.1080/00275514.2021.1872869.
DOI: https://doi.org/10.1080/00275514.2021.1872869

Interpretive Summary: Efforts to repopulate the endangered Hawaiian plantain plant known as "ale" have been greatly hindered by destructive downy mildew disease outbreaks since 2014. However, the identity of the water mold that causes Hawaiian ale downy mildew is not known, which makes it difficult to develop effective disease control measures. In a collaborative effort between USDA-ARS, USDA-APHIS, and the Hawai'i Department of Agriculture, this research identified the downy mildew pathogen as a previously undescribed species. This new pathogen is different from all other known species that infect plants in the plantain family. A new name was given to the pathogen, and a diagnostic key was developed so that scientists and conservationists can identify the mold in the future. The identification of the downy mildew pathogen of the endangered Hawaiian ale plant is the first step towards developing control measures that can be specifically targeted against the pathogen.

Technical Abstract: Plantago princeps var. princeps is an endangered native Hawaiian plant, and part of the recovery plan includes re-population using plants grown in a nursery. However, disease pressure from downy mildew is hindering re-population efforts. The organism associated with the downy mildew was determined to be a Peronospora species with brown, ellipsoid conidia measuring 21 by 16 µm on average, which was morphologically different from validly described species of Peronospora that infect Plantago species, but it was morphologically similar to the invalidly published species Peronospora lanceolatae [Art. 40.1]. Comparison of mitochondrial cytochrome oxidase subunit I (cox1), mitochondrial cytochrome oxidase subunit II (cox2), nuclear internal transcribed spacer (ITS), nuclear 28S rRNA D1-D2 (28S) loci revealed the unknown Peronospora to be molecularly divergent from Peronospora alta and Peronsopora plantaginis, but very similar to a Peronospora specimen on Plantago lanceolata, the type host of Peronospora lanceolatae. Phylogenetic trees inferred with maximum likelihood and Bayesian inference from a concatenated alignment and individual gene trees confirmed the divergence of the unknown Peronospora from P. alta and P. plantaginis and its similarity to P. lanceolatae. However, attempts to inoculate Plantago lanceolata with the strain from Plantago princeps var. princeps were unsuccessful, which, in conjunction with divergence in ITS, suggests that the unknown Peronospora is specific to Plantago princeps var. princeps. Herein, the Peronospora strain on Plantago princeps var. princeps is described as the new species Peronospora kuewa based on morphology, molecular phylogenetics, and host specificity. In addition, Peronospora gaponenkoae is described here to honor Nina Ivanova Gaponenko on the basis of her description of P. lanceolatae.