Where flowers bloom, so do downy mildews: new species and new records of Hyaloperonospora, Peronospora and Plasmopara species on ornamental and wild plants in the United States

Authors: Salgado-Salazar, Catalina; RODRIGUEZ-SALAMANCA, LINA - Virginia Tech; ROMBERG, MEGAN - Animal And Plant Health Inspection Service (APHIS); DAVIS, WILLIAM - Davey; MCCONNELL, MEGAN - Bartlett Tree Service; HAVEN, AMBROSIA - The Ohio State University; PEDUTO HAND, FRANCESCA - The Ohio State University

Submitted to: Plant Health Progress
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/11/2025
Publication Date: N/A
Citation: N/A

Interpretive Summary: Downy mildews cause some of the most severe and destructive diseases of ornamental plants, impacting both the yield and the quality of the final product. However there are many knowledge gaps concerning their identity. This study used DNA sequences and microscopy to describe several first observations of downy mildew diseases on cultivated and wild ornamental plants in the U.S. Additionally one previously unknown species on ornamental snapdragon was found. The results of this study have important implications for production and disease management. The information from this study will be useful to plant pathologists, regulatory officials and others involved in the management of plant health in the ornamental plant industry.

Technical Abstract: Downy mildews are biotrophic plant pathogens of high value crops which can cause significant damage and economic losses. The ornamental and floriculture industries are particularly susceptible to downy mildews, especially when annuals, perennials, and woody plants are growing in protected environments. In addition to affecting cultivated plants, downy mildews are a common occurrence on wild relatives, or non-cultivated plant hosts. The role these play, however, as pathogen reservoirs and on the emergence or re-emergence of downy mildew epidemics is still unknown. In this study, we report downy mildew disease observations on several annual, perennial and woody shrub plants in several states in the Eastern U.S. Based on morphological observations and phylogenetic analyses of two nuclear loci (ITS, rnLSU) and two mitochondrial loci (cox2, cox1), we report Peronospora and Plasmopara occurrences on frost grape (Ohio), Lenten rose (North Carolina, Virginia), mock strawberry (Maryland), redbud (North Carolina), viburnum (North Carolina) and Virginia creeper (Maryland, Ohio). Further analysis indicated that the Peronospora isolates on snapdragon (Antirrhinum sp.) from Virginia are clearly separated from other P. antirrhini isolates collected on Misopates orontium, indicating they constitute a separate species here described as Pe. ceperoae. The phylogenetic analyses also supported the combination of Peronospora matthiolae, a pathogen of Matthiola incana (stock), into Hyaloperonospora, based on specimens collected in Ohio. Collectively, our results suggest a wider geographic distribution for the downy mildew species reported and highlight the need for continuous surveillance of these pathogens to prevent outbreaks and their introduction to naïve environments.