Species of Rhododendron and resulting hybrids are very important hosts of the quarantine pathogen Phytophthora ramorum, since they play a major role in the spread of the pathogen in Europe. However, many other Phytophthora species infect these hosts, causing similar symptoms. Widmer (4) listed 17 Phytophthora species as foliar pathogens of rhododendron in the United States. A survey was conducted in Greece in October 2009, in which potted plants of Rhododendron spp. were inspected for symptoms of necrotic lesions on leaves and buds caused by P. ramorum. Symptomatic plants were observed in one of the nurseries inspected in the Triphylia Region in southwestern Peloponnese. Isolations from symptomatic leaves on PARBhy-V8 selective agar medium (1) yielded Phytophthora isolates. Colonies on V8 juice agar appeared white and cottony, with a radial growth of 4.2 to 4.6 mm per day at 28°C with a maximum growth temperature of 36°C. Sporangia were produced abundantly on the medium surface and in water; the sporangia were broadly ovoid and papillate and 35 to 50 × 25 to 35 μm. Chlamydospores, 25 to 40 μm in diameter, were observed in 2-week-old cultures, while no sexual structures were observed. Three of the isolates examined were identified as P. nicotianae B. de Haan on the basis of morphological and physiological features (3,4). Genomic DNA was extracted from pure cultures of an isolate and the internal transcribed spacer (ITS) region was amplified using the ITS4/5 primer pair. Sequence analyses by BLAST indicated that the isolates were most similar to P. nicotianae (GenBank Accession No. AJ 854295.1) with sequence identity values of 99%. One of the isolates was deposited in the culture collection of the University of Athens (ATHUM 6519). Detached wounded leaves of Rhododendron hybrid cv. Red Jack were inoculated with agar plugs. Necrotic lesions, similar to those observed in the nursery, appeared on the inoculated leaves after 7 days of incubation at 26°C, while no symptoms developed on control leaves inoculated with sterile agar plugs. The pathogen was consistently reisolated from infected leaves, but not from the controls. P. nicotianae, being a thermophilic species, is the most common Phytophthora species in Greece, reported on more than 30 plant species (2). This pathogen has been reported on Rhododendron spp. in the United States (3,4), but to our knowledge, there was no record of this pathogen on these hosts as yet in Europe and this is the first published report of the pathogen on Rhododendron in Greece.
References: (1) A. Belisario et al. Plant Dis. 87:101, 2003. (2) K. Elena. Technical Bulletin No 13. Benaki Phytopathological Institute. Athens, Greece (in Greek), 1999. (3) D. C. Erwin and O. K. Ribeiro. Phytophthora Diseases Worldwide. The American Phytopathological Society, St. Paul, MN, 1996. (4) T. L. Widmer. Online publication. doi: 10.1094/PHP-2010-0317-01-RS, Plant Health Progress, 2010.