Phytophthora ramorum sp nov., a new pathogen on Rhododendron and Viburnum

Since 1993, a hitherto unidentified Phytophthora species has been found ass ociated with twig blight disease in Rhododendron and, sporadically, Viburnu m. The morphology and growth characteristics of fourteen isolates from Germ any and the Netherlands were investigated, together with their breeding sys tem, the internal transcribed spacer (ITS) regions of the ribosomal DNA, am plified fragment length polymorphism (AFLP) fingerprints, and isozyme profi les, which were compared to those of a number of outgroup species. Morpholo gically the isolates are characterized by abundant production of chlamydosp ores and elongate, ellipsoid, deciduous sporangia with a short pedicel, in which they resemble P. palmivora, However, sporangia were semi-papillate, c hlamydospores were much larger and cardinal temperatures much lower than th ose of P. palmivora. Oogonia with amphigynous antheridia and plerotic oospo res were produced in dual cultures with an A2 mating type strain of P. cryp togea. ITS1 and ITS2 sequences of the unidentified species were closest to those of P. lateralis, but differed in three and eight nucleotides respecti vely from the latter species, AFLP fingerprints and isozyme patterns of mal ate dehydrogenase (MDH-2) and malic enzyme (MDHP) showed that the isolates formed a homogeneous group, distinct from all examined outgroup species, in cluding P. lateralis. It was concluded that they represent a new Phytophtho ra species, described here as P. ramorum sp. nov. In pathogenicity tests al l isolates of P. ramorum were pathogenic to Rhododendron.