First report of downy mildew (Plasmopara obducens) on impatiens walleriana in Italy

During summer 2012, potted plants of impatiens (Impatiens walleriana) grown in gardens near Biella (northern Italy) showed symptoms of downy mildew. Infected leaves were paler green than normal and showed white, downy growth on the lower surface. Plants collapsed very rapidly, especially at high relative humidity (RH). Microscopical observations of infected leaves, maintained for 24 h at high RH, disclosed the presence of hyaline, tree-like, straight, 120-350×5.8-10 μm sporangiophores, with three sterigma. Sporangia were ovoid, hyaline and 10.7-15.4×11.7-16.6 (average 13.2×14.5) μm in size. Oospores were not observed in leaf tissue. The DNA region encoding the large ribosomal subunit (LSU rDNA) was amplified using primers NL1 and NL4 (Maier et al., 2003) and sequenced (GenBank accession No. JX880252LSU). BLAST analysis of the 729 bp product obtained showed a similarity of 99% (E-value=0) with Plasmopara obducens from the USA (GenBank accession No. JX217746). To confirm pathogenicity, 60-day-old impatiens plants, grown singly in 15 litre pots in a growth chamber at 20±1°C, were inoculated by spraying leaves with a suspension of 1×105 sporangia/ ml. Control plants were sprayed with distilled water. Plants were covered with plastic bags for 4 days. The first symptoms (chlorosis) developed 8 days post inoculation. Control plants remained healthy. This is the first report of P. obducens in Italy. The disease has been reported in several countries, including the USA (Wegulo et al., 2004), UK (Lane et al., 2005) and Serbia (Bulajic et al., 2011). Currently, this disease is present in several gardens in northern Italy, where its importance may increase rapidly due to the widespread cultivation of impatiens.