Phytophthora ramorum causes shoot and foliar blight on Rhododendron spp., Viburnum spp. (4), Pieris spp. (2), Kalmia latifolia, and Camellia spp. in several European countries (1--4). In December 2002, we received diseased C. japonica growing in containers from several nurseries in Galicia (northwestern Spain). These young camellia plants had leaves with brown-to-black, water-soaked lesions with diffuse borders that expanded into larger blotches resulting in dead leaves and necrotic lesions on the petioles. Eventually the entire plant wilted and died. Tissue from the leading edge of the lesions was transferred to a selective medium (V8 agar supplemented with pimaricin (10 μg/ml), rifampicin (25 μg/ml), hymexazol (5 μg/ml), and benomyl (5 μg/ml)) and incubated for 3 to 4 days at 20°C in the dark. A Phytophthora sp. was isolated, transferred to carrot piece agar (CPA) (4), and incubated in alternating light. Isolates exhibited coralloid mycelium with concentric rings and a radial growth of 2.5 to 3 mm per day at 20°C. The hyaline-to-yellowish chlamydospores were terminal and intercalary, occasionally lateral, and 24 to 74 μm in diameter. The caducous, sympodial, semipapillate sporangia had a length/breadth ratio of 1.8 to 2.1 and a short pedicel (<5 μm) or no pedicel. Oogonia, antheridia, and oospores were produced by pairing the isolates with P. cryptogea A2 tester BBA 63651 (3,4) provided by S. Werres. Oogonia were subspherical and smooth-walled, antheridia were amphyginous, and oospores were plerotic. The internal transcribed spacer-rDNA polymerase chain reaction (PCR) product obtained by using DNA extracted from mycelium and nested PCR with P. ramorum-specific primers was the size reported for P. ramorum (1). Pathogenicity of the isolates was confirmed by inoculating detached leaves of C. japonica. Five isolates were tested on leaves from 15 young plants growing in containers. Three leaves of each plant were detached and inoculated with each isolate. Leaves were dipped for 5 min into a suspension of sporangia and mycelial fragments and maintained at 20°C and 95% humidity. After 15 days, lesions that developed from the petiole base exceeded 25 mm, and the pathogen was consistently reisolated from the lesions. Leaves inoculated with water from sterile CPA plates did not develop symptoms. A C. japonica isolate has been deposited in the Spanish Type Culture Collection (CECT 20519). To our knowledge, this is the first report of P. ramorum on C. japonica in Spain, though the pathogen has been isolated from Rhododendron spp. and Viburnum tinus growing in several nurseries in Galicia.
References: (1) J. M. Davidson et al. On-line publication doi:10.1094. Plant Health Progress, PHP, 2003-0707-01-DG, Plant Management Network. (2) A. J. Inman et al. First Report of Ramorum Dieback (Phytophthora ramorum) on Pieris in England. On-line publication. New Dis. Rep. Vol. 7, British Society for Plant Pathology, 2003. (3) E. Moralejo et al. Plant Dis. 86, 9:1052, 2002. (4) S. Werres et al. Mycol.Res.105:1155, 2001.
