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

Abstract

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, prior to 2002, it was recorded only as a dieback of Rhododendron (Ericaceae) and Viburnum (Caprifoliaceae). However, its potential threat to native trees led to the introduction of emergency European Community (EC) phytosanitary measures. In November 2002, CSL received PHSI samples from a few mature specimens of Pieris formosa var. forrestii (syn. P. forrestii) cv. ‘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 and dark bronze lesions on the petiole end of leaves, often extending down the midrib, or on their tip or edge. Stem sections were surface-decontaminated in running tap water in quarantine facilities for 4 h. Sections from the leading edge of lesions were transferred to a semiselective medium (P5ARP[H]; Lane et al., 2003). 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 A positive PCR reaction was also obtained using P. ramorum-specific primers (K.J.D. Hughes, unpublished data). 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.G. Linderman, USDA-ARS, personal communication, 2002).