First Report of Peronospora digitalidis causing downy mildew disease on foxglove in Oregon

Authors: WALLACE, EMMA - Orise Fellow; Crouch, Jo Anne

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/10/2017
Publication Date: 11/13/2017
Citation: Wallace, E., Crouch, J. 2017. First Report of Peronospora digitalidis causing downy mildew disease on foxglove in Oregon. Plant Disease. 102:827. https://doi.org/10.1094/PDIS-09-17-1504-PDN.

Interpretive Summary: Plants worldwide are continually damaged and sometimes killed by downy mildew diseases. This research describes a novel outbreak of downy mildew disease affecting crops of greenhouse-grown foxglove plants. The foxglove disease was found in a commercial nursery in Oregon. This research is significant because this is the first time the downy mildew was ever found in Oregon, and only the second sighting ever made from the United States. This research will be useful to extension personnel, growers, plant pathologists, and quarantine officials who work to control downy mildew diseases in the United States.

Technical Abstract: Foxglove (Digitalis purpurea) is a biennial plant in the Plantaginaceae family. Widely cultivated as a striking flowering ornamental plant, foxglove is also a source of toxic cardiac glycosides that may be fatal if ingested at high doses, or used pharmaceutically to treat human heart failure. 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 5% of the foxglove plants to be lost. Pathogen hyphal growth on the abaxial leaf surface created a fuzzy appearance. Sporangiophores were hyaline, dichotomously branched, 258.6 to 687.3 µm long x 7.4 to 13.9 µm wide. Sporangia were pale brown, ellipsoidal to oval, 24.5 to 32.1 x 18.4 to 23.1 µm. No oospores were present. The morphological characters were consistent with Peronospora digitalidis (Gumann 1923), the only species reported to cause downy mildew disease of foxglove (Farr & Rossman 2017). DNA sequence identification was performed for the 28S large ribosomal DNA subunit, the nuclear ITS rDNA, and the mtDNA cox2 using primer pairs LROR/LR7, ITS1-0/LR-O, and Cox2-F/Cox2-RC4 as described (Vilgalys & Hester 1990; Rivera et al. 2016). 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 speciose (MF372422). There were no ITS sequences for P. digitalis in GenBank, but the highest sequence similarities (96-97%) 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 Pe. acuminatus MF372507, P. flava on Linaria vulgaris AY198245, Peronospora sp. on Gambelia speciose MF372422). Similarly, the LSU sequence (MF996741) had 96% identity to P. silvestris (AY035490) and P. aquatica (AY271991), which affect Plantaginaceae plants Veronica urticifolia and V. anagaliis-aquatica, respectively. In lieu of pathogenicity tests, morphological traits, host incidence, and nucleotide sequence similarity support identification of the causal agent of the foxglove downy mildew as P. digitalidis. This disease was first observed in the U.S. on the coast of central California in 2002 (Tjosvold & 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 foxglove plants have to be pruned to remove disease damage, it can negatively alter the shape of the plant. Furthermore, if the disease is not identified and managed early, it can result in expensive losses.