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

Abstract

Norwegian Institute for Agricultural and Environmental Research, Plant Health and Plant Protection Division, Hogskoleveien 7, 1432A˚s, NorwayInAugust2008,diseasedImpatienswallerianaplantswerereceivedfromaprivategardeninOslo,southeasternNorway.Symptomsontheadaxialside of leaves were pale yellowing without distinct lesions. White downygrowth appeared on the abaxial side and on flower buds. Infected leaveswilted and abscised from the stem (Toppe et al., 2008). Microscopicexamination of the pathogen revealed hyaline, monopodial sporangio-phoreswithbranchesarrangedatrightangles.Sporangiawereovoidandhyaline. Oospores were observed in dried diseased tissue of stems, leaf-stalks and petals of flower buds. On the basis of symptoms and morpho-logical characteristics, the pathogen was identified as Plasmoparaobducens. This species is commonly described on wild and cultivatedImpatiens and can be easily distinguished from the other known cause ofdowny mildew in impatiens, Bremiella sphaerosperma (Constantinescu,1991; Cunnington et al., 2008). Partial nuclear large ribosomal subunitDNAsequences(650 bp)wereobtainedusingPCRandprimersNL1andNL4 as described by Maier et al. (2003), and found to be identical tosequences 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 inpublic parks in Oslo and Bergen (southwestern Norway). In 2009, thedisease was found outdoors at several places in southeastern Norway(also on cv. Xtreme), 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 plantswith a suspension of sporangia, covering with plastic for the first 24 hafter inoculation, and then maintaining in a research greenhouse (20 C,80–95% RH). Symptoms of downy mildew developed after 14 daysthatmatched those originally observed, together with the morphology of there-isolated pathogen. This is the first observation of downy mildewcaused by P. obducens on I. walleriana in Norway. Earlier, the diseasewas described outdoors on I. noli-tangere, a native plant in Norway(Jorstad,1964).Sofar,itisnotknownwhether theobservedpathogenofI. walleriana is the same as the pathogen previously observed in nativeImpatiensspeciesinNorway.References