First report of ramorum leaf blight and dieback ( Phytophthora ramorum ) on Camellia spp. in the UK

The recently described pathogen Phytophthora ramorum (Werres et al., 2001) is causing leaf blight and dieback on a range of plant species, including rhododendron (Werres et al., 2001), viburnum (Lane et al., 2003) and Pieris formosa (Inman et al., 2003) in parts of Europe, and tree mortality of Lithocarpus densiflorus and Quercus spp. in the USA (Rizzo et al., 2002). Due to the potential threat to European flora from P. ramorum, EC emergency legislation was introduced in November 2002. During the winter of 2003, 18 separate occurrences of P. ramorum were found on Camellia spp. from 15 nurseries and three from public gardens across the UK. The majority of findings were from southern England; however, infected camellia plants were also found in Wales, north-west England and Guernsey, Channel Islands. On camellia, P. ramorum principally causes a leaf blight, although on rare occasions it has also been found to infect petioles and flower buds, and can cause a limited dieback of shoots and stems. Leaf lesions are dark brown with a diffuse, often chlorotic, margin. These can occur anywhere on the leaf lamina but are most commonly found progressing from the leaf tip or margin. Lesions are variable in size, ranging from small necrotic spots, a few milli-metres in diameter, to an extensive necrosis covering most of the leaf surface. Infected leaves usually fall prematurely. The pathogen was isolated by aseptically excising leaf tissue from the leading edge of the necrotic lesion, surface-decontaminating in running water and plating onto a Phytophthora-specific semiselective medium ((P5ARP[H]) (Lane et al., 2003). Isolates typically grew at the rate of 2 mm day−1 and consisted of coenocytic hyphae with a coralloid appearance. Sporangia were sympodially formed, deciduous with a short pedicel (< 5 µm), semipapillate and measured 20–28 µm × 42–70 µm with a length to width ratio of 1:2·1. Chlamydospores were hyaline to slightly pigmented and measured 30–70 µm diameter. These morphological features conform to the published description of P. ramorum (Werres et al., 2001). The identity of the cultures was confirmed by TaqMan® PCR using species-specific primers. Koch's postulates were confirmed by inoculating healthy camellia leaves in situ with a camellia isolate of P. ramorum and re-isolating the pathogen from the resulting symptomatic tissue. No water control inoculations produced any leaf symptoms. This is the first finding of P. ramorum infecting camellia.