First report of Ramorum dieback (Phytophthora ramorum) on Hamamelis virginiana in the UK

Phytophthora ramorum causes sudden oak death on native Lithocarpus and Quercus, as well as damage on other shrubs and trees in California, USA. In Europe, it is recorded mainly as a dieback of Rhododendron (Ericaceae) and Viburnum (Caprifoliaceae). In the UK, the disease has also occurred on other ornamentals such as Pieris (Inman et al., 2003), container grown yew (Taxus baccata) (Lane et al., 2004) and Camellia spp. (Beales et al., 2004). Its potential threat to native trees has led to the introduction of emergency EC phytosanitary measures.

In August 2003, the PHSI collected a sample from a mature soil grown specimen of Hamamelis virginiana (Virginian witch-hazel; Hamamelidaceae) from a large public garden in South Wales, where P. ramorum on rhododendron was under eradication. The symptoms on Hamamelis were brown lesions on the leaves, often on the leaf tip or edge (Fig. 1), sometimes delineated by large and small leaf veins. Twigs were also affected, resulting in an aerial dieback (Fig. 2). Stem and leaf sections were surface decontaminated and sections from the leading edge of lesions transferred to a semi-selective medium (P5ARP[H]; Lane et al., 2002). A Phytophthora species 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 mating type was determined as A1 due to the formation of typical sexual structures when crossed with a known A2 mating type of P. ramorum from USA. The ITS sequences were identical to those of P. ramorum on the NCBI GenBank database (Accession No. AY540491). A positive TaqMan^® PCR reaction was also obtained using primers and probes designed to identify P. ramorum (K. Hughes, unpublished).

Pathogenicity of the isolate was confirmed by wound-inoculating healthy leaves of Hamamelis virginiana and Rhododendron catawbiense with mycelial plugs of the Hamamelis isolate and incubating at room temperature (c. 20° C), on the laboratory bench for six days. Extensive lesions developed on the leaves and the pathogen was re-isolated from the leading edge of the Hamamelis lesions; thus 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 Hamamelis virginiana in the UK. The plants were destroyed and measures were taken to eradicate the pathogen according to EC phytosanitary legislation and the EC was notified. Since this initial finding, there have been no further reports of P. ramorum on Hamamelis virginiana in the UK.

Acknowledgments

We thank Mr. T Davies for his help in confirming the identity of the host and for technical assistance with sampling.