Pseudoperonospora cubensis causing downy mildew disease on Impatiens irvingii in Cameroon: a new host for the pathogen

In December 2007, leaves of Impatiens irvingii infected by downy mildew were collected in Bertoua (Department of Lom et Djérem, East Province), Cameroon. In the initial stage of infection, the pathogen caused epiphyllous blood red spots on the leaves which at first had indistinct margins, but were later enclosed with dark red boundaries. In extreme cases the downy mildew led to curling of leaves and necrosis of plant tissues (Fig. 1).

On the underneath of the infected leaf spots, sporangiophores typical of Pseudoperonospora emerged in groups from the stomata. The sporangiophores were 3-6 times monopodially branched, 300-530 µm high, with a trunk 145-330 µm long and 6.5-11.5 µm wide at the base which was slightly inflated or not at all (Fig. 1A-C). The slightly to strongly sinuous ultimate branchlets of the sporangiophores were 2.5-22 µm long and 2-3 µm wide (Fig. 1D). The dark brown sporangia were ellipsoidal to oblong and measured 24.5-31 × 14.5-21 µm, with a l/w ratio of 1.4-1.8, and a distinct, hyaline papilla (Fig. 1E-G). These data are in perfect agreement with those given for Pseudoperonospora cubensis by Choi et al. (2005). Cross-sections through infected host tissue stained with Congo red showed that the sporangiophores were connected to knobby-vesicular, aseptate, intercellular mycelium typical of Pseudoperonospora (Fig. 1H). Specimens are deposited at HUYI, KRAM and WU herbaria.

To confirm the identification of the pathogen, DNA was extracted from infected plant tissue, using the CTAB protocol of Riethmüller et al. (2002). The complete ITS rDNA region was amplified and sequenced using the primers ITS5-P2 (Voglmayr & Constantinescu, 2008) and ITS4, and deposited in GenBank (AccessionNo. EU660054). It differed by only one base substitution in the ITS2 region from P. cubensis sequences AY608616, AY608617, AY608619 and DQ409815 (sequence identity of 99.8%) and was placed into the P. cubensis clade with 100% posterior probability in a Bayesian analysis of the sequence data of Choi et al. (2005) (Fig. 3). This is the first report of Pseudoperonospora on Impatiens (Balsaminaceae). This confirms the polyphagous nature of P. cubensis (Choi et al., 2005) as its host range extends from Cucurbitaceae and Cannabaceae to Balsaminaceae. Besides Plasmopara obducens and Pl. constantinescui (Voglmayr & Thines, 2007), Pseudoperonospora cubensis is the third downy mildew recorded on Impatiens and of potential concern for Impatiens growers.