First outbreak of Phytophthora ramorum in England, on Viburnum tinus

Phytophthora ramorum is a recently described pathogen causing oak mortality in California, USA and is commonly known as sudden oak death (Werres et al., 2001; Rizzo, et al., 2002). The disease has reached epidemic proportions in the central coast area causing death of native oaks (Lithocarpus and Quercus) as well as damage on a range of other native plants. In Europe it has not been recorded on oaks, but has been found causing twig blight of species of Rhododendron, and occasionally stem base decay of Viburnum, in Germany and the Netherlands (Werres et al. 2001). Although, P. ramorum is therefore already found in parts of Europe there is concern about its threat to forestry and native plants, in addition to other sectors of the horticultural industry. Therefore, in 2002, emergency phytosanitary measures were implemented to prevent spread within the European Community.

In February 2002, Defra’s Plant Health and Seeds Inspectors submitted one plant from a batch of five container-grown plants of Viburnum tinus cv 'Eve Price', from a garden centre in southern England, showing severe aerial dieback, stem base discoloration and partial root decay, to the Central Science Laboratory (CSL) for diagnosis. The plants had recently been obtained from another English nursery. Samples from the basal region were excised, then surface decontaminated by washing in running tap water, flowing into quarantine drainage, for 12 hours. They were then transferred to a semi-selective medium (P5ARP[H]) and floated in Petri’s mineral solution (Jeffers & Martin, 1986). Following five days incubation, a Phytophthora sp. was consistently isolated with features typical of P. ramorum (Werres et al., 2001). The culture was slow growing and produced weakly corolloid mycelium. Numerous semi-papillate, deciduous, sympodial sporangia were observed in addition to hyaline to light brown, large chlamydospores. No sexual structures were seen. Additionally, the ITS sequence, when compared to data on the National Centre for Biotechnology Information molecular database (http://www.ncbi.nlm.nih.gov) was identical to P. ramorum. The Forestry Commission Research Agency confirmed the identity by reference to type cultures and subsequently determined the mating type as A1. To date, the A1 mating type has only been found in Europe whilst the A2 has only been found in the USA. Pathogenicity of the isolated fungus was confirmed using the Rhododendron cv ‘Catawbiense Grandiflorum’ twig dipping method as described by Werres et al. (2001); re-isolation of the fungus satisfied Koch’s postulates.

Following the confirmatory diagnosis, statutory plant health action was taken. All five plants and associated growing media were destroyed, contaminated surfaces disinfected and other susceptible hosts inspected. The finding was reported to the European Community Plant Health Standing Committee. Additional inspection and testing at the garden centre found no further evidence of the pathogen. The nursery that supplied the plants was intensively inspected and material sent to CSL for diagnosis. This revealed the presence of one block of 100 container-grown plants V. tinus cv 'Eve Price' with two plants infected with P. ramorum. However, a further block of 300 container grown plants and six stock plants in the open ground of the same variety as well as 20 container-grown rhododendron plants tested negative for P. ramorum. The source of the infection at this nursery is unknown. Statutory plant health action, as described previously, was taken, and no further symptoms have been observed on either premise.

This is the first report of Phytophthora ramorum causing dieback of Viburnum tinus and the first record of the pathogen for England.