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

Authors: Salgado-Salazar, Catalina; ROMBERG, MEGAN - Animal And Plant Health Inspection Service (APHIS); HUDELSON, BRIAN - University Of Wisconsin

Submitted to: Fungal Systematics and Evolution
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/29/2023
Publication Date: 9/22/2023
Citation: Salgado-Salazar, C., Romberg, M.K., Hudelson, B. 2023. Plasmopara echinaceae, a new species of downy mildew affecting cone flowers (Echinacea purpurea) in the United States. Fungal Systematics and Evolution. 12:203-217.
DOI: https://doi.org/10.3114/fuse.2023.12.10

Interpretive Summary: Downy mildew diseases are widespread, pervasive and can result in damage so great that the host plant’s value is reduced and/or not marketable, yield is reduced, or the host is killed outright. Downy mildew diseases caused by Plasmopara species are some of the most severe and destructive diseases of ornamental plants, impacting the quality of product, making it a major constraint for the ornamental industry. This study highlights the existence of different species of Plasmopara causing downy mildews in cultivated Echinacea purpurea (Purple coneflower) using molecular and microscopic characteristics. Downy mildew species can move to new hosts and sometimes emerge in economically important agrosystems. Although the potential impact the new species described have on the ornamental industry is still unknown, this information will be used by extension personnel, plant breeders, plant pathologists and others engaged in the management of plant diseases.

Technical 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 the tribes Asteroideae, Chicorideae and Heliantheae. However, with the use of molecular DNA characters for phylogenetic studies, distinct lineages of P. halstedii in the Asteraceae have been identified and segregated as new species. During August of 2022, a downy mildew was observed on potted Echinacea purpurea grown in a retail greenhouse in Jefferson , WI. 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, 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.