First report of Ramorum dieback (Phytophthora ramorum) on Pieris in England

A. J. Inman¹*, V.C. Townend¹, A.V. Barnes¹, C. R. Lane¹, K. J. D. Hughes¹, R. L. Griffin¹ and S. J. Eales²

¹ Plant Health Group, Central Science Laboratory (CSL), Department for Environment, Food & Rural Affairs (Defra), Sand Hutton, York, YO41 1LZ, U.K
² Plant Health and Seeds Inspectorate (PHSI), Defra, 10-11 City Business Centre, Basin Road, Chichester, West Sussex, PO19 8DU, U.K.

*a.inman@csl.gov.uk

Accepted for publication 07/03/03

Phytophthora ramorum causes sudden oak death in California, USA on native Lithocarpus and Quercus, as well as damage on other shrubs and trees. In Europe, it is recorded only as a dieback of Rhododendron (Ericaceae) and Viburnum (Caprifoliaceae). However, its potential threat to native trees led to the introduction of emergency EC phytosanitary measures.

In November 2002, CSL received PHSI samples from a few mature specimens of Pieris formosa var. forrestii (syn. P. forrestii) cultivar ‘Wakehust’ (Ericaceae) from a private garden, open to the public, at which P. ramorum was under eradication on Rhododendron. The symptoms on Pieris were an aerial twig dieback (Fig. 1) and dark bronze lesions on the petiole end of leaves, often extending down the midrib, or on their tip or edge (Fig. 2). Stem sections were surface-decontaminated in running tap water in quarantine facilities for 4 hours. Sections from the leading edge of lesions were transferred to a semi-selective medium (P₅ARP[H]; Lane et al., 2002). A Phytophthora sp. with characteristics typical of P. ramorum (Werres et al., 2001) was consistently isolated. The cultures were slow growing with weakly coralloid mycelium, numerous semi-papillate, deciduous, sympodial sporangia and hyaline to light brown, large chlamydospores. The ITS sequences were identical to those of P. ramorum on the NCBI Genbank database (http://www.ncbi.nlm.nih.gov). A positive PCR reaction was also obtained using P. ramorum-specific primers (K. Hughes, unpublished). Colonies of P. ramorum also developed from leaf lesions plated directly onto P₅ARP[H]-agar.

Pathogenicity of the isolate was confirmed by wound-inoculating healthy leaves of P. formosa var. forresti and Rhododendron catawbiense with mycelial plugs; extensive lesions developed on the leaves. The pathogen was re-isolated from the leading edge of the Pieris lesions, completing Koch’s postulates. Healthy leaves inoculated with agar alone, as negative controls, did not develop symptoms.

This is the first report of P. ramorum on Pieris in Europe. The plants were destroyed and the EC notified. The susceptibility of this genus was predicted in laboratory bioassays (Inman et al., 2002; Linderman et al., 2002). Pieris japonica is also reported as a natural host in the USA (R. Linderman, pers. comm.).

References

Inman AJ, Beales PA, Lane CR and Brasier CM, 2002. Comparative pathogenicity of European and American isolates of Phytophthora ramorum to leaves of ornamental, hedgerow and woodland under-storey plants in the UK. In: Proceedings of the Sudden Oak Death Science Symposium: The State of Our Knowledge. 15–18 December 2002. Monterey, California, USA [http://danr.ucop.edu/ihrmp/sodsymp/posters.html].

Lane CR, Beales PA, Hughes KJD, Griffin RL, Munro D, Brasier CM and Webber JF, 2002. First outbreak of Phytophthora ramorum in England, on Viburnum tinus. New Disease Reports [http://www.bspp.org.uk/ndr/] Volume 6.

Linderman RG, Parke JL, Hansen EM, 2002. Relative virulence of Phytophthora species, including the sudden oak death pathogen, P. ramorum, on leaves of several ornamentals. Phytopathology 92 (6), Supplement: S47.

Werres S, Marwitz R, Man in’t Veld WA, De Cock AWAM, Bonants PJM, De Weerdt M, Themann K, Ilievea E, Baayen RP, 2001. Phytophthora ramorum sp. nov., a new pathogen on Rhododendron and Viburnum. Mycological Research 105, 1155-1165.

The British Society for Plant Pathology