First report of Impatiens downy mildew (Plasmopara obducens) in the UK

In early June 2003, a sample of Impatiens walleriana Firefly Series from southern England was found with symptoms of a downy mildew. Infected leaves appeared paler green than normal, with a white, downy growth developing on lower surfaces. Symptoms were initially inconspicuous but lower leaf surfaces quickly became covered with fungal growth and premature leaf fall was common. Stunting of plants and poor flowering were also observed. Collapse of plants could be very rapid, especially under humid conditions, resulting in up to 80% losses. Two downy mildews have been described on Impatiens: the more commonly described Plasmopara obducens (syn. Peronospora obducens) on wild and cultivated Impatiens spp. (Sohi & Tyagi, 1974) and the less well-known fungus Bremiella sphaerosperma (Constantinescu, 1991). Detailed examination of a representative sample (IMI 390985) revealed the sporangiophores were monopodially branched at the base, individual branches did not arise in a distinct pattern, apical branchlets were at right angles to the main axis and there was no apical thickening; all characteristics of P. obducens. The identification was supported by DNA sequencing. The 5' end of the nuclear DNA coding for the large ribosomal subunit (LSU rDNA) was amplified by PCR using the primers NL1 and NL4, according to Maier et al. (2003). The sequence of the amplified product was obtained and deposited in GenBank (AY587558). Comparison of the sequence of the LSU rDNA region with other sequences available in the GenBank database revealed that it was almost identical, with only one base pair substitution, to P. obducens found on I. capensis in the USA (AY035522).

Following the initial discovery, the Plant Health and Seeds Inspectorate (Defra) carried out a survey of growers resulting in several further findings of P. obducens on a range of cultivars, in addition to samples from commercial growers, civic floral displays, private gardens and growers from around the UK. PHSI inspections revealed no evidence of downy mildew on wild impatiens around nurseries. There are no previous reports of a downy mildew on wild or cultivated Impatiens spp. in the UK (Francis & Waterhouse, 1988), although P. obducens has been found in other parts of Europe as well as Canada, USA, Guatemala, Central Asia, China, India, Korea and Russia (D. Jones, pers. comm.). Statutory plant health action continues to be taken against this regulated disease in the UK.