Scottish Agricultural Science Agency (SASA), East Craigs, Edinburgh EH12 8NJ, UK
Accepted: 18 Feb 2005
Within statutory surveys for Phytophthora ramorum, a Rhododendron ponticum with wilting foliage and blackened shoot tips, from a nursery in Perthshire, was examined in September 2003. A homothallic Phytophthora spp. was isolated on selective V8 medium (PARPNH) (Jung et al., 1996), showing a weak stellate pattern with sparse aerial mycelium. The antheridia were paragynous, some of them strikingly large or twisted (Fig. 1).
Hardly any sporangia were produced on V8 agar, but did develop abundantly in water and were semipapillate and ovoid, ellipsoid or obturbinate in shape (Fig. 2). Later in the year, the same Phytophthora was isolated from another rhododendron from an adjoining nursery and from a Gaultheria shalon and a Vaccinium vitis-idaea with leaf lesions and dieback symptoms from a nursery in Stirlingshire.
Detached leaves of Gaultheria shalon (Fig. 3) and Rhododendron ponticum were inoculated with mycelium plugs of the new isolates from Gaultheria and Rhododendron, respectively. After 4 days at 18-20°C and with a 10 h day-light cycle, typical lesions started to develop from which the fungus was re-isolated, fulfilling Koch's postulates. Pathogenicity on Vaccinium vitis-idaea has not yet been confirmed.
The ITS1, 5.8S subunit, and ITS 2 region of the ribosomal RNA gene of all isolates was sequenced (GenBank accession numbers AY879291-879294) and compared with ITS sequences of known Phytophthora species obtained from GenBank. The closest matches (98% identity) were with Phytophthora inflata and Phytophthora citricola. Sequencing of the cytochrome C oxidase (cox) subunit I gene, after amplification with primers CoxF4N and CoxR4N (Kroon et al., 2004) confirmed P. inflata (AY894684-894687).
P. inflata was first described by Caroselli & Tucker (1949), causing a pit canker on elm trees in Michigan. Since this first description, the pathogen has only once been reported in the UK (Hall et al., 1992), isolated from roots of Sambucus tenuifolium and Syringa vulgaris. As reference, one of these isolates (IMI 342899 from Sambucus) was compared to the new isolates, showing the same morphological features. The distinct â€œirregularly shaped swollenâ€ hyphae Hall observed on Lima bean agar were also observed on V8 agar (Fig. 4).
Caroselli NE, Tucker CM, 1949. Pit canker of elm. Phytopathology 39, 481-488.
Hall G, Dobson S, Nicholls C, 1992. First record of Phytophthora inflata in the United Kingdom. Plant Pathology 41, 95-97.
Jung T, Blaschke H, Neumann P, 1996. Isolation, identification and pathogenicity of Phytophthora species from declining oak stands. European Journal of Forest Pathology 26, 253-272.
Kroon LPNM, Verstappen ECP, Kox LFF, Flier WG, Bonants PJM, 2004. A rapid diagnostic test to distinguish between American and European populations of Phytophthora ramorum. Phytopathology 94, 613-620.
©2005 The Authors