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First Report of Peronospora digitalidis Causing Downy Mildew Disease on Foxglove in Oregon

    Authors and Affiliations
    • E. C. Wallace , Mycology and Nematology Genetic Diversity and Biology Laboratory, USDA-ARS, Beltsville, MD 20705, and Oak Ridge Institute for Science and Education, Oak Ridge, TN 37830
    • J. A. Crouch , Mycology and Nematology Genetic Diversity and Biology Laboratory, USDA-ARS, Beltsville, MD 20705.

      Foxglove (Digitalis purpurea) is a biennial plant in the plantain family (Plantaginaceae). Widely cultivated as a striking flowering ornamental plant, foxglove is also a source of toxic cardiac glycosides that may be used pharmaceutically to treat human heart failure but are fatal if ingested at high doses. In the spring of 2017, foxglove plants with angular chlorotic and necrotic lesions characteristic of downy mildew disease were observed in a commercial greenhouse in Washington County, OR. The disease was identified and treated early but still caused losses of 5% to the foxglove crop. Pathogen hyphal growth on the abaxial leaf surface created a fuzzy appearance. Sporangiophores were hyaline, dichotomously branched, 258.6 to 687.3 µm long × 7.4 to 13.9 µm wide. Sporangia were pale brown, ellipsoidal to oval, 24.5 to 32.1 × 18.4 to 23.1 µm. No oospores were present. The morphological characters were consistent with Peronospora digitalidis (Gäumann 1923), the only species reported to cause downy mildew disease of foxglove (Farr and Rossman 2017). DNA sequence identification was performed for the 28S large ribosomal DNA subunit, the nuclear internal transcribed spacer (ITS) rDNA, and the mtDNA cox2 using primer pairs LROR/LR7, ITS1-O/LR-O, and Cox2-F/Cox2-RC4 as described (Rivera et al. 2016; Vilgalys and Hester 1990). BLASTn queries of NCBI GenBank showed the cox2 sequence (MF996739) had 99% identity with P. digitalidis (KJ654207). The ITS sequence from the sample (MF996740) showed 97% identity to Peronospora sp. on Penstemon acuminatus (MF372507), 96% identity to P. flava on Linaria vulgaris (AY198245), and 96% identity to Peronospora sp. on Gambelia speciosa (MF372422). There were no ITS sequences for P. digitalidis in GenBank, but the highest sequence similarities (96 to 97%) to the ITS sequences from the sample (MF996740) were all causal agents of downy mildew disease of plants in the same family as foxglove (Peronospora sp. on Penstemon acuminatus MF372507, P. flava on Linaria vulgaris AY198245, Peronospora sp. on Gambelia speciosae MF372422). Similarly, the LSU sequence (MF996741) had 96% identity to P. silvestris (AY035490) and P. aquatica (AY271991), which affect Veronica urticifolia and V. anagallis-aquatica, respectively, both of which are also in the plantain family. In lieu of pathogenicity tests, morphological traits, host incidence, and nucleotide sequence similarity support identification of the causal agent of the foxglove downy mildew (BPI910529) as P. digitalidis. This disease was first observed in the United States on the coast of central California in 2002 (Tjosvold and Koike 2002), but to our knowledge, this is the first report of downy mildew on foxglove in Oregon. Downy mildew disease on foxglove has the potential to cause significant economic strain on the greenhouse and nursery industry. Chemical control of downy mildew can be costly, and if many disease-damaged foxglove leaves have to be removed, plant appearance can be compromised. If the disease is not identified and managed early, it can result in expensive losses.