The destructive invasive pathogen Phytophthora lateralis found on Chamaecyparis lawsoniana across the UK

Summary In 2010–2011, Phytophthora lateralis was isolated from diseased Chamaecyparis lawsoniana exhibiting dieback and mortality at eight geographically separate forest, parkland and shelterbelt locations in England, Scotland and Northern Ireland. In 2011, P. lateralis was also isolated from young symptomatic nursery plants of C. lawsoniana and Thuja occidentalis recently imported into Scotland from mainland Europe. These are the first findings of P. lateralis in the UK. At six of the field sites, only collar and root lesions were observed. However, at two sites, large stem and branch lesions unconnected to the collar region were also observed. Phytophthora lateralis was readily isolated from both aerial and basal lesions. In artificial inoculation experiments, two Scottish isolates of the pathogen caused lesions on C. lawsoniana shoots and were readily reisolated from the lesions, their pathogenicity being comparable to that of P. lateralis isolates originating from outside the UK. Isolates from six field sites and the two nursery interceptions exhibited ITS and cox II sequences identical to published sequences of French and North American isolates. However, the isolates from two field sites shared an ITS sequence with Taiwanese isolates and differed from North American, French and Taiwanese isolates by a single‐base substitution in cox II, suggesting a separate evolutionary history. It is clear that P. lateralis now presents a significant threat to C. lawsoniana in Britain. The main source of the outbreaks is likely to be imported infested nursery stock.