|Rivera, Yazmin - Rutgers University|
|Salgado-salazar, Catalina - Rutgers University|
|Creswell, Thomas - Purdue University|
|Ruhl, Gail - Purdue University|
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/5/2015
Publication Date: 6/1/2016
Citation: Rivera, Y., Salgado-Salazar, C., Creswell, T.C., Ruhl, G., Crouch, J. 2016. First report of downy mildew caused by Peronospora sp. on Agastache in the United States. Plant Disease. 100(6):1249.
Interpretive Summary: Downy mildew diseases are caused by fungal-like water molds that result in significant damage to many crop plants in the United States. Hyssops are popular flowering herbaceous perennial plants, prized for their brilliant flowers, aromatic fragrance and their ability to attract pollinators in the landscape and garden. This research describes an outbreak of downy mildew disease on hyssop plants at a commercial nursery in Michigan during 2015. The disease was caused by a water mold previously unknown to science. This is the first time this disease of hyssops caused by this mold was ever seen in the United States. Knowledge of this disease and new pathogen will be useful to plant regulatory officials working to control the spread of downy mildew diseases in the United States, and provides information about this new disease to hyssop growers and gardeners.
Technical Abstract: Hyssops (Agastache spp. and cultivars) are fragrant perennial plants in the mint family (Lamiaceae), primarily native to North America. Hybrid Agastache are bred as ornamental plants, valued for their showy flowers and aromatic scents that attract pollinators to the landscape. In May 2015, a commercial nursery in Berrien County, Michigan reported downy mildew symptoms on seed-grown Agastache ‘Bolero’, including chlorotic leaf spots on the adaxial leaf surface that became necrotic with age, and gray sporangial masses on the abaxial leaf surface. These symptoms on Agastache cultivars were also experienced in 2014 at the same nursery, resulting in the loss of 1,248 plants grown from plugs. Microscopic examination of diseased foliar tissue showed straight hyaline sporangiophores, monopodially branched, ending with curved branchlets bearing single sporangia. Sporangia were ellipsoid with a light brown coloration. Sporangiophores measured 251-500 µm in length, while sporangia measured on average 23.8 × 18.9 µm (20.3-25.9 × 17.2-20.3 µm; n=20). The morphological characteristics were consistent with members of the genus Peronospora, but insufficient for species-level diagnosis. Genomic DNA from infected leaf tissue was extracted using the DNeasy Plant Mini kit (QIAGEN, Gaithersburg, MD). The ITS region of the rDNA and the cox2 region of the mtDNA were PCR amplified and sequenced bi-directionally using primers ITS-O/LR-O and Cox2-F/Cox2-RC4, respectively (Thines et al. 2009). Resultant sequences (NCBI GenBank accession nos.: ITS=XX00000 cox2=XX00000) were compared to those in NCBI GenBank using BLASTn. The ITS sequence shared 99% nucleotide identity with the P. belbahrii holotype (host=Ocimum basilicum) and a P. belbahrii specimen on Agastache sp. from the U.K. The cox2 sequence shared 99% nucleotide identity with a specimen of P. glechomae (host=Glechoma hederaceae) and the P. elsholtziae holotype (host=Elsholtzia splendens). Besides P. belbahrii, the only other Peronospora species previously reported from Agastache is P. lopanthii (syn.=P. lamii); however, these reports are limited to checklists rather than scientific literature (Farr and Rossman 2015) and pre-dated the application of molecular analyses. The specimen examined herein is morphologically and genetically distinct from the P. belbahrii holotype by having smaller sporangia and sporangiophores, and a different cox2 sequence, but also genetically distinct from P. lopanthii. Furthermore, the Peronospora specimens infecting Agastache in the U.K. were identified using ITS similarity (Henricot et al. 2010), which our cox2 data suggest as inadequate for species identification. We therefore consider the Peronospora in our report as an undetermined species, closely related to P. belhbarii but discrete. Our data and previous research reflect the need for comprehensive taxonomic revision of the Lamiaceae-infecting Peronospora. To our knowledge, this is the first report of this distinct Peronospora species causing downy mildew on Agastache in the U.S. This pathogen may present a threat to the production of these ornamental plants and the North American ecosystems in which native species reside.