Phytophthora ramorum on three new hosts detected using on-site diagnostics

Phytophthora ramorum causes dieback of Rhododendron and Viburnum. In the UK, this pathogen has been reported on a range of ornamentals from different plant families e.g. Pieris (Inman et al., 2003) and Hamamelis (Giltrap et al., 2004).

Hughes et al. (2005) reported successful on-site testing for P. ramorum by real-time (TaqMan) PCR using a Cepheid Smart Cycler. Identical on-site testing also identified P. ramorum on Magnolia stellata, Magnolia loebneri and Griselinia littoralis. The pathogen caused leaf infections on Magnolia (Fig. 1) and Griselinia. Lesions were usually brown to black, typically occurring at the tip or leaf margins but able to spread over the whole leaf. Replicate material was sent to CSL for further testing and P. ramorum was consistently isolated from stem and leaf tissue following surface decontamination and isolation onto semi-selective medium (Lane et al., 2002). The ITS sequences for the P. ramorum isolated (M. stellata DQ376175, M. loebneri DQ376176 and Griselinia AY924256) was identical to other P. ramorum isolates recorded on the GenBank database.

Pathogenicity of each isolate was confirmed by wound-inoculating healthy plants of Magnolia stellata, M. loebneri and Griselinia littoralis, and incubating these in a damp chamber at room temperature (c. 20°C) for six days. Control healthy plants were either wounded or wound-inoculated with agar alone. These did not develop symptoms, whereas extensive lesions developed on the pathogen-inoculated leaves (Fig. 2). P. ramorum was re-isolated from the leading edge of infection, thereby completing Koch’s postulates.

This is the first report of P. ramorum on Magnolia stellata, M. loebneri and Griselinia littoralis 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.

Acknowledgments

We thank Mr. T Davies for his help in confirming the identity of the host and for technical assistance with sampling. The work forms part of the PHD Defra – funded survey to determine the incidence of Ramorum dieback.