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

P.A. Beales^1*, T. Brokenshire², A.V. Barnes¹, V.C. Barton¹ and K.J.D. Hughes¹

¹ Central Science Laboratory (CSL), Sand Hutton, York, YO41 1LZ, UK
² Plant Protection Services, Committee for Horticulture, Raymond Falla House, PO Box 459, Longue Rue, St Martins, Guernsey GY1 6AF

*p.beales@csl.gov.uk

Accepted for publication 12/02/04

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, eighteen 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, northwest England and Guernsey.

Figure 1: Phytophthora ramorum symptoms on camellia. (a) Leaf tip necrosis, (b) Necrotic spots over leaf lamina, (c) Necrotic lesion extending from leaf margin

On camellia P. ramorum principally causes a leaf blight, although on rare occasions has also been found to infect petioles, 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 (see Fig. 1 a-c). Lesions are variable in size ranging from small necrotic spots, a few millimetres in diameter, to an extensive necrosis overing 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 semi-selective medium ((P₅ARP[H]) (Lane et al., 2003). Isolates typically grew at the rate of 2mm/day and consisted of coenocytic hyphae with a coralloid appearance. Sporangia were sympodially formed, deciduous with a short pedicel (<5 µm), semi-papillate and measured 20–28 µm x 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.

References

Inman AJ, Townend VC, Barnes AV, Lane CR, Hughes KJD, Griffin RL, Eales SJ, 2003. First report of ramorum dieback (Phytophthora ramorum) on Pieris in England Plant Pathology 52, 785.

Lane CR, Beales PA, Hughes KJD, Griffin RL, Munro D, Brasier CM, Webber JF, 2003. First outbreak of Phytophthora ramorum in England, on Viburnum tinus. Plant Pathology 52, 414.

Rizzo DM, Garbelotto M, Davidson JM, Slaughter GW, Koike ST, 2002. Phytophthora ramorum as the cause of extensive mortality of Quercus spp. and Lithocarpus densiflorus in California. Plant Disease 86, 205– 214.

Werres S, Marwitz R, Man in’t Veld WA, De Cock AWAM, Bonants PJM, De Weerdt M, Themann K, Ilievea E Baayen RP, 2001. Phytophthora ramorum sp. nov., a new pathogen on rhododendron and viburnum. Mycological Research 105, 1155–1165.

The British Society for Plant Pathology