First Report of Downy Mildew on Nursery-Grown Hellebore Caused by Peronospora pulveracea in California

Hellebore or Lenten rose (Helleborus × hybridus) is an evergreen, herbaceous perennial in the family Ranunculaceae. Hellebores are sold as decorative, potted plants and as shade-loving landscape plants favored for their attractive and prolonged blooms in late winter or early spring. In April of 2008, downy mildew-like growth was observed on the foliage of approximately 60 containerized plants of Helleborus 'Blue Lady', 'Pink Lady', 'White Lady', and 'Royal Heritage' grown outdoors in a retail nursery in coastal San Mateo County, California. Infected foliage had angular, vein-delimited, dark brown-to-black speckled lesions on adaxial leaf surfaces turning dry and necrotic with age. Young leaves were small and distorted. Affected flowers were spotted and brown. The abaxial sides of affected leaves had light brown-to-purplish downy mildew-like growth. Subhyaline conidia, globose to ellipsoid in shape, ranged from 25 to 31 × 17 to 24 μm (average 28 × 21 μm). Conidiophores ranged from 265 to 375 × 5 to 11.5 μm (average 333 × 8.9 μm), branching dichotomously four to five times in the upper half. Morphological measurements fell within the range previously described for Peronospora pulveracea and P. alpicola, which were reported on Helleborus spp. and Ranunculus aconitifolius, respectively (1,2). DNA sequence of the internal transcribed spacer region of rDNA of our isolate (Genbank Accession No. FJ384778) matched sequences of P. pulveracea (Genbank Accession No. AY198270) and P. alpicola (Genbank Accession No. AY198271) with 100% identity. These two organisms are taxonomically indistinguishable by rDNA sequences and are likely to be the same species (3). To our knowledge, this is the first report of P. pulveracea on Helleborus × hybridus in California and the United States. Lenten rose is commercially propagated by seed, which is a potential pathway for introduction of this pathogen. Mature plants are sold and shipped intra- and interstate as decorative flowering plants or nursery stock. The importance and economic impact of this disease is limited, but significant economic losses could occur during production. References: (1) E. A. Gäumann. Beitr. Kryptogamenflora Schweiz 5:113, 1923. (2) G. Hall. Mycopathologia 126:57, 1994. (3) H. Voglmayr. Mycol. Res. 107:1132, 2003.