Peronospora bergamotii, Hyaloperonospora daughtreyae and H. iberidis: new species associated with downy mildew diseases affecting ornamental plants in the United States

Authors: Salgado-Salazar, Catalina; LEBLANC, NICHOLAS - Orise Fellow; WALLACE, EMMA - Orise Fellow; DAUGHTERY, MARGERY - Cornell University; Crouch, Jo Anne

Submitted to: European Journal of Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/8/2020
Publication Date: 5/15/2020
Citation: Salgado-Salazar, C., Leblanc, N., Wallace, E., Daughtery, M., Crouch, J. 2020. Peronospora bergamotii, Hyaloperonospora daughtreyae and H. iberidis: new species associated with downy mildew diseases affecting ornamental plants in the United States. European Journal of Plant Pathology. 157(1):311-326. https://doi.org/10.1007/s10658-020-01989-9.
DOI: https://doi.org/10.1007/s10658-020-01989-9

Interpretive Summary: Destructive downy mildew diseases are an increasingly difficult problem for numerous horticultural and crop plants. Many are newly discovered, which means that the identity of the disease-causing organism is not known. This research used DNA sequences and physical traits to identify and give names to three new species of downy mildew molds infecting four widely grown floriculture plants, namely bee balm, spider flower, hyssops and candytuft. This research is significant because it is the first time that the organisms causing these diseases have been identified. This is also the first time that a downy mildew disease has ever been found on bee balm, a fragrant ornamental plant that provides the flavorful ingredient used in herbal bergamot teas. This new information will allow plant pathologists, horticulturalists and diagnostic laboratories to accurately identify these new plant pathogens. These findings will also be useful to plant regulatory officials working to control the spread of downy mildew diseases and provide important information to gardeners, landscapers, and commercial growers.

Technical Abstract: Species in the genera Hyaloperonospora and Peronospora (Oomycota, Peronosporaceae) are obligate biotrophic pathogens responsible for destructive downy mildew diseases of ornamental horticulture plants worldwide. Throughout the past decade, newly emergent downy mildew diseases have become increasingly prevalent, but often the identity of the pathogen is unknown or poorly defined. In this study we set out to identify the downy mildew pathogens infecting three widely grown ornamental plants: Agastache spp. (hyssops), Tarenaya hassleriana (syn. Cleome hassleriana; spider-flower) and Monarda didyma (bee balm or bergamot). Phylogenetic analysis of the internal transcribed spacer rDNA and cox2 revealed two new species, which are described here as Peronospora bergamotii infecting both Monarda didyma and A. mexicana, and Hyaloperonospora daughtreyae infecting T. hassleriana. One sample of Peronospora collected from Agastache sp. ‘Bolero’ did not cluster with P. bergamotii, but its relationship to other species in the clade was not sufficiently resolved for taxonomic determination. Phylogenetic analysis also supported the combination of Peronospora iberidis, a pathogen of Iberis spp. (candytuft), into Hyaloperonospora. To assess intrasample ITS sequence variation from P. bergamotii, data were generated using high-throughput amplicon sequencing (HTAS). Multiple haplotypes were identified from the P. bergamotii samples, with less than 1% intrasample variation observed. From a core set of haplotypes shared across the HTAS the variant frequencies could not be completely explained by heterozygosity alone.