First Report of Phytophthora ramorum on Rhododendron sp. in Spain
Authors: Moralejo E. , Werres S. 
Published: May 2007

Phytophthora ramorum has been recently described (4) from isolates collected since approximately 1993, from Rhododendron and Viburnum in the Netherlands and Germany. Since 2001 and 2002, respectively, there have been unconfirmed reports from Poland and Great Britain (1). There are growing concerns about this oomycete since it has been recognized as the causal agent of sudden oak death in California, where thousands of oaks and tanoaks (Quercus agrifolia, Q. kelloggii, Q. parvula var. shrevei, and Lithocarpus densiflorus) have died. Since the first outbreak in Marin County, California in 1995, (3) the epidemic has remained concentrated mainly along the Pacific Coast near San Francisco but has been reported in Oregon (2). There have been no reports of oaks or other trees affected in Europe. During February 2002, E. Moralejo surveyed several nurseries in Mallorca, (Balearic Islands, Spain) to verify the presence of P. ramorum on rhododendron. Two shipments of rhododendrons, from the towns of Palma and Santa Maria del Camí, showed twig blight and necrotic spots at leaf tips and margins. Less than 0.5% of the aerial parts of the potted plants of both sets were affected. Isolations from symptomatic tissue consistently yielded cultures of Phytophthora using selective media (corn meal agar supplemented with nystatin, rifampicin, and ampicillin). Pure cultures were obtained from three isolates by hyphal-tip transfers on carrot piece agar (CPA) and identified as P. ramorum on the basis of morphological and cultural characters (4). Colonies grown on CPA at 17°C were appressed with sparse aerial mycelium and formed typical concentric growth rings under 16 h fluorescent light. Cardinal temperatures for vegetative growth were 2°C (minimum), 20°C (optimum), and 28°C (maximum). Daily radial growth was 3 to 3.2 mm at 20°C. Sporangia formed singly or in clusters on aerial sporangiophores in the absence of free water, and were mainly semipapillate, caducous, with a very short or no pedicel, ellipsoidal to elongated-ovoid, with a length/breath ratio of 1.9 to 2.1. Large spherical, terminal, and intercalary chlamydospores, 32 to 72 μm in diameter, were readily formed in agar. The mating type of these isolates from Mallorca was A1 based on dual cultures on CPA with four different heterothallic Phytophthora species. Crossing was successful with P. cryptogea BBA 63651 mating type A2. Oogonia were subglobose and measured 24 to 32 μm in diameter. Oospores were plerotic. Antheridia were always amphigynous, and mainly barrel-shaped to spherical, 14 to 22 μm in diameter One of the three isolates also produced a very small number of gametangia with the A2 tester strain P. cinnamomi BBA 62660, but the oospores appeared degenerate. Only A1 mating type strains of P. ramorum have been found in Europe. The Mallorca isolates have been deposited in the culture collection of the Federal Biological Research Centre for Agriculture and Forestry (BBA), Germany. To our knowledge, this is the first report of P. ramorum in Spain. Further research is urgently needed to assess whether the Mediterranean evergreen oak ecosystem is in danger.

References: (1) EPPO Reporting Service. Published on-line, http://www.eppo.org/PUBLICATIONS/EPPO_RS/reporting_service.html Publication no. 2002/040 and 2002/077. (2) E. M. Goheen et al. Plant Dis. 86:441, 2002. (3) D. M. Rizzo et al. Plant Dis. 86:205, 2002. (4) S. Werres et al. Mycol. Res. 105:1155, 2001.