First report of Phytophthora ramorum on container-grown plants of rhododendron and viburnum in Slovenia

Phytophthora ramorum is a recently described pathogen which attacks Rhododendron and Viburnum spp., in addition to other ornamental plants in Europe (Inman et al. 2003; Werres et al., 2001). It is also well known as the cause of sudden oak death in North America (Rizzo et al., 2002).

In 2003, plants of Viburnum and Rhododendron species were inspected at nurseries, garden centres, parks and gardens throughout Slovenia, as part of emergency phytosanitary measures. In July, symptoms similar to those reported for P. ramorum were observed on container-grown plants of R. catawbiense 'Grandiflorum' and V. farreri in two garden centres in Ljubljana and Viburnum x bodnantense in a nursery in Gorenjska region. The rhododendron plants showed twig dieback; leaf blight, wilting and discoloration at the stem base occurred on both Viburnum species. Excised symptoms were examined at the Agricultural Institute of Slovenia after surface sterilisation with 70% ethanol for 10s and rinsing with sterile distilled water. Samples were placed aseptically onto plates of P5ARP (Jeffers & Martin, 1986), from which pure cultures of P. ramorum were obtained. The identity of the fungus was confirmed by morphological and cultural characters (Werres et al., 2001). Sporangia were 37-79 (57) x 17-32 (27) µm, with a length to width ratio from 1.8 to 2.5 (2.1). Daily radial growth on carrot piece agar (CPA) at 20°C was 3.1 to 3.2 mm. Two isolates, one from each host genus, were compared but no substantial differences were found. Central Science Laboratory confirmed representative isolates as P. ramorum on morphological characteristics and positive PCR reactions were obtained using P. ramorum-specific primers.

Two isolates, one from Rhododendron and one from Viburnum were tested for pathogenicity on leaves of Rhododendron catawbiense 'Cunninghams white'. Mycelial plugs of diameter 8 mm, taken from 10 day-old CPA cultures were placed on on the underside of five leaves scored with a scalpel. CPA discs without mycelium were applied in a similar manner. The wound-inoculated leaves were kept in a growth chamber at 20⁰C and 100% humidity. After five days, necrotic lesions were seen only on leaves inoculated with P. ramorum. The fungus was successfully reisolated eight days after inoculation. No necrotic lesions developed on control leaves.