Phytophthora hedraiandra occurs on rhododendron in Slovenia

In July 2004 and July 2005, necrotic lesions on shoots were found, with dieback and wilting in severe cases, on established rhododendron bushes in a public park near Kranj, central Slovenia. Necrotic lesions were wiped with cotton dipped in 70 % ethanol. Marginal tissue was excised and incubated in the dark at 20°C on P5ARP agar plates. A similar isolate was obtained in both years, which was then inoculated within four days on cornmeal agar (CMA) and incubated at 25°C. The cultures grew around 6 mm daily. A few papillate, subglobose sporangia appeared after two weeks, occurring abundantly when mycelial plugs were submerged in pond water. Oogonia were 29 -33 µm (31 µm) and oospores were 24-29 µm (26 µm); amphigynous antheridia were infrequent. The morphological characters resembled those described for Phytophthora cactorum or P. hedraiandra.

Sequences of the isolates (ITS rDNA, spanning ITS 1 and 2 plus the 5.8S rDNA) were generated using primers ITS5 and ITS4 in primary PCR (35 cycles: 96°C, 1 min, 55°C 1 min, 72°C 2 min) and in sequence reactions. Isolates obtained in 2004 and 2005 yielded identical sequences and were identifical to the ex-type strain of P. hedraiandra, CBS 111725 (AY707987). A representative isolate (CBS 119903) was deposited at the Centraalbureau voor Schimmelcultures in the Netherlands. The ITS rDNA of this isolate was deposited at GenBank (DQ507201).

Pathogenicity testing was performed using inoculated 10 days-old mycelial plugs of CBS 119903 grown on CMA into wounds of potted R. catawbiense cv. ‘Cunninghams white’. Sterile plugs were used on control plants. All plants were kept for two weeks at 20°C. Extensive necrotic lesions developed on inoculated shoots, expanding on to petioles and leaves. Controls showed no symptoms. Phytophthora hedraiandra was reisolated from margins of lesions.

Phytophthora hedraiandra occurs on Viburnum spp. in the Netherlands (de Cock & Lévesque, 2004), Spain (Moralejo et al., 2005) and Italy (Belisario et al., 2005). It causes wilting, stem cankers and root and collar rots. It has also recently been reported from various rhododendron cultivars with leaf lesions and shoot dieback in the USA (Schwingle et al., 2006). This is the first report of Phytophthora hedraiandra in Slovenia and the first European record on Rhododendron spp.