First report of Plasmopara obducens (downy mildew)on Impatiens walleriana in Norway

In August 2008, diseased Impatiens walleriana plants were received from a private garden in Oslo, southeastern Norway. Symptoms on the adaxial side of leaves were pale yellowing without distinct lesions. White downy growth appeared on the abaxial side and on flower buds. Infected leaves wilted and abscised from the stem (Fig. 1) (Toppe et al., 2008). Microscopic examination of the pathogen revealed hyaline, monopodial sporangiophores with branches arranged at right angles (Fig. 2). Sporangia were ovoid and hyaline. Oospores were observed in dried diseased tissue of stems, leafstalks and petals of flower buds (Fig. 3). On the basis of symptoms and morphological characteristics, the pathogen was identified as Plasmopara obducens. This species is commonly described on wild and cultivated Impatiens and can be easily distinguished fromthe other known cause of downy mildew in impatiens, Bremiella sphaerosperma (Cunnington et al., 2008). Partial nuclear large ribosomal subunit DNA sequences (650 bp) were obtained using PCR and primers NL1 and NL4 as described by Maier et al. (2003),and found to be identical to sequences of several P. obducens isolates deposited in GenBank (AY035522.1, AY587558.1, EF196869.1).

Severe disease attacks were later observed in 2008 on cv. Xtreme Red in public parks in Oslo and Bergen (southwestern Norway). In 2009, the disease was found outdoors at several places in southeastern Norway (also on cv. Xtreme Red), and at two different greenhouse-sites on cv. Silhouette grown from cuttings and imported earlier the same year. Koch’s postulates were fulfilled on cv. Xtreme Red by inoculating plants with a suspension of sporangia, covering with plastic for the first 24 hours after inoculation, and then maintaining in a research greenhouse (20° C, 80-95% RH). Symptoms of downy mildew developed after 14 days that matched those originally observed, together with the morphology of the re-isolated pathogen. To our knowledge, this is the first observation of downy mildew caused by P. obducens on I. walleriana in Norway. Earlier, the disease was described outdoors on I. noli-tangere, a native plant in Norway (Jørstad, 1964). So far, it is not known whether the observed pathogen of I. walleriana is the same as the pathogen previously observed in native Impatiens species in Norway.