Plasmopara echinaceae, a new species of downy mildew affecting cone flowers (Echinacea purpurea) in the United States

Abstract

Downy mildew is one of the most important diseases of commercial sunflower and other Asteraceae hosts, including ornamental Rudbeckia. Plasmopara halstedii has historically been identified as the causal agent of this disease, considered a complex of species affecting nearly 35 genera in various tribes. However, with the use of molecular DNA characters for phylogenetic studies, distinct lineages of P. halstedii in the Asteraceae have been identified, confirmed as distinct or segregated as new species. During August of 2022, a downy mildew was observed on potted Echinacea purpurea grown in a retail greenhouse in Jefferson County, Wisconsin, USA. Phylogenetic analyses of the cytochrome c oxidase subunit 2 (cox2) and nuclear large subunit ribosomal RNA (nc LSU rDNA) gene regions indicated these Plasmopara sp. isolates are not conspecific with P. halstedii. Based on phylogenetic evidence and new host association, the Plasmopara isolates from E. purpurea are here described as Plasmopara echinaceae. Diagnostic morphological characters for this new species were not observed when compared with other isolates of P. halstedii or other Plasmopara species found on Asteraceae hosts, and therefore a list of species-specific substitutions in the cox2 region are provided as diagnostic characters. As this study corresponds to the first observation of downy mildew in cone flowers, it is recommended to follow the required disease prevention guidelines to prevent outbreaks and the establishment of this plant pathogen in production sites.