A new threat to UK heathland from Phytophthora kernoviae on Vaccinium myrtillus in the wild

The recently described Phytophthora kernoviae (Brasier et al., 2005) has caused significant impact on ornamental and tree species since 2003 mainly in South West England, with fewer findings in South Wales, Cheshire and more recently Scotland (Anon 2008). It has also been detected on Annona cherimola and from woodland soil in New Zealand (Anon 2006).

UK heathland contains a diverse range of flora and fauna of national and international importance, comprising 75% (upland) and 20% (lowland) of the world's heath resource (biodiversity action plans http://www.ukbap.org.uk/habitats.aspx). Concerns for this sensitive habitat were raised recently when P. kernoviae was detected causing severe leaf necrosis and stem dieback on Vaccinium myrtillus (bilberry) in semi-natural ancient woodland in Cornwall. Infected plants were found in two areas each approximately 1m². Other plants such as Rhododendron ponticum were also infected with P. kernoviae nearby. Further findings have subsequently been confirmed on V. myrtillus heathland on an exposed hill 210 m above sea level, with no obvious other susceptible hosts in the near vicinity. Stem and leaf blackening were the principal symptom, however petiole necrosis and stem dieback were also evident. Samples were submitted by the PHSI to CSL and were tested according to an ISO 17025 accredited protocol.

The isolate was morphologically typical for P. kernoviae consisting of coralloid mycelium; sympodial semi-papillate sporangia, (30 - 40 µm x 22 - 30 μm); amphigynous antherida; plerotic oospores (22 - 26 µm); chlamydospores were absent. The diagnosis was confirmed by Taqman PCR and sequencing of ITS regions. (Isolate is held in CSL culture collection: accession no. 2565, GenBank Accession No. EU442181.) Koch's postulates were proven using healthy V. myrtillus plants by inoculating the underside of wounded and unwounded attached leaves with a 6mm plug of the original isolate (unaffected agar medium was used as a control). Inoculated plants were bagged and incubated under conditions conducive for P. kernoviae. Symptoms on both wounded and unwounded leaves were evident after approximately seven days (Fig. 1). Control inoculations remained healthy. The organism was re-isolated, thereby completing Koch's postulates.

A recent Defra funded project (http://www.defra.gov.uk/planth/kernovii/kernrep.pdf) completed by CSL investigated the susceptibility of heathland plants to P. kernoviae. Results indicated that V. myrtillus, V. vitis-idaea and Arctostaphylos uva-ursi are all highly susceptible to the pathogen. This is the first wild finding of this statutory listed organism on V. myrtillus in the world. Eradication procedures are underway and further surveys of heathland plants are in progress.