First report of Peronospora sp. causing downy mildew disease on Geum sp. in the northeastern United States

Authors: WALLACE, EMMA - Orise Fellow; DAUGHTREY, MARGERY - Cornell University; RANE, KAREN - University Of Maryland; SALGADO-SALAZAR, CATALINA - Orise Fellow; Crouch, Jo Anne

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/18/2018
Publication Date: 1/19/2018
Citation: Wallace, E., Daughtery, M., Rane, K., Salgado-Salazar, C., Crouch, J. 2018. First report of Peronospora sp. causing downy mildew disease on Geum sp. in the northeastern United States. Plant Disease. 102(7):1463. https://doi.org/10.1094/PDIS-09-17-1503-PDN.
DOI: https://doi.org/10.1094/PDIS-09-17-1503-PDN

Interpretive Summary: Downy mildew diseases cause extensive damage to crop plants. This research describes two downy mildew disease outbreaks on two popular new cultivars of the flowering ornamental plant geum at commercial nurseries in Maryland and New York. Almost all of the affected plants were killed by the disease. This research used DNA and visual criteria to identify the water mold that caused the disease. This research is significant because this is first time this deadly disease is documented from the eastern U.S., and the first time downy mildew has been reported on Geum in the U.S. in several decades. This research will be useful to plant regulatory officials, diagnostic clinics, extension agents, growers and gardeners.

Technical Abstract: Plants in the genus Geum are clump-forming perennials in the family Rosaceae. Known for long bloom times and the ability to attract pollinators, Geum hybrids are popular ornamental plants commonly grown in borders and rock gardens (Brickell & Cathey, 2004). In early April 2017, the entire crop of Geum ‘Fire Storm’ plants from a commercial nursery in Baltimore County, MD was affected with downy mildew disease. Downy mildew symptoms were also extensive, with over 90% of the plants affected, on Geum ‘Fireball’ in June 2017 in a commercial nursery in Suffolk County, NY. Foliage displayed vein-bounded, angular chlorotic lesions, and necrosis as the disease progressed. The abaxial leaf surface displayed “downy” gray patches. Microscopic examination of diseased tissue revealed dichotomously branched sporangiophores measuring 380 to 917 µm (avg.=626 µm; n = 17, with the first ramification at 70% of sporangiophore length. Sporangia were light brown, globose to broadly ellipsoidal, 16.7 to 21.7 x 14.7 to 19.9 µm;(avg.= 19.5 x 17.5 µm; n = 44). No oospores were observed. Morphological characteristics were consistent with both Peronospora gei and P. potentillae, two species previously described as causal agents of Geum downy mildew (Gäumann 1923). The mtDNA cox2 and the nuclear rDNA ITS were sequenced bi-directionally from PCR amplicon from sporulating leaf lesions extracted with the Omni Prep DNA extraction kit (G-Biosciences, St. Louis, MO; Rivera et al. 2016). BLASTn queries of NCBI GenBank showed cox2 sequences of the samples shared 97% and 96% identity with Peronospora species affecting Rosaceae hosts. ITS sequences from the samples had 100% identity to P. potentillae-repantis and P. potentillae-anserinae infecting Potentilla species (KT795476, KT795475). There were no ITS sequences of P. gei and P. potentillae available through GenBank. A combined cox2/ITS maximum likelihood tree from the Geum samples and several Peronospora species infecting Rosaceae hosts (Petrželová et al. 2016) revealed the Geum downy mildew samples clustered as a distinct species from related Peronospora species (100% bootstrap support). Based on the combined morphological and molecular evidence, there is sufficient data to identify the Geum downy mildew pathogen as a member of the genus Peronospora, but available taxonomic resources cannot provide identification of the samples to the species level. There has not been a comprehensive study of Peronospora in the family Rosaceae using molecular data, limiting our understanding of the impact of downy mildew species on Geum and related crops. To our knowledge, this is the first report of Peronospora and downy mildew disease affecting Geum sp. in the northeastern United States, with previous listings of the disease in the U.S. limited to the Midwest and the West Coast (Farr and Rossman 2017). It is crucial to understand the causal agents of Geum downy mildew and their geographic range to aid in improving management strategies.