A further threat to UK heathland bilberry (Vaccinium myrtillus) by Phytophthora pseudosyringae

In November 2007, Phytophthorakernoviae was reported causing a significant threat to heathland bilberry (Vaccinium myrtillus) (Beales et al., 2009). While surveying for this statutory pathogen, a different Phytophthora sp. was isolated from diseased V. myrtillus plants in an area of ancient woodland in Staffordshire (Fig 1). Symptoms included dark brown to black necrotic lesions on the stems (often observed as bands) and top down dieback of plants (Fig. 2), which were similar to those caused by P. kernoviae.

Isolations on carrot piece agar (CPA) resulted in colonies that were appressed with minimal aerial hyphae, with sporangia produced sparsely on all solid media tested, but abundantly when small pieces of colonised CPA were floated for 24 hours in distilled water. Variably caducous, limoniform to ovoid sporangia were semi-papillate, 41.3 x 27 µm (mean), forming on simple sympodial sporangiophores.Catenulate hyphal swellings were frequently present, but no chlamydospores observed. The organism was homothallic with plerotic oogonia, 29.2 µm in diameter (mean), regularly forming on CPA. Antheridia were predominantly paragynous.Morphological features conformed to P. pseudosyringae (Jung et al., 2003). The culture was stored in the Fera collection (Accession No. cc2655).

The ITS 1 and 2 regions of the nrRNA gene were sequenced from five isolates taken from separate plants. All sequences were identical and matched with a single dimorphic site of the consensus sequence to P. pseudosyringae type isolate (GenBank AY230190 [Jung et al., 2003]). Koch’s postulates were performed on non-wounded stems of five-year-old whole plants. A small piece of 10-day-old inoculum grown on CPA was placed on the stem and wrapped in damp cotton wool and Parafilmä. Sterile CPA was used as a control. The plant was covered with a polythene bag and incubated at20^o C for 10 days, after which necrotic lesions were observed. P. pseudosyringae was consistently isolated from the newly formed lesions. No symptoms were observed on the control plants.

P. pseudosyringae was first identified causing root and collar rot of deciduous tree species in parts of Europe (Jung et al., 2003), and on beech, hornbeam and Nothofagus in the UK (S. Denman, Forest Research, Alice Holt Lodge, Farnham GU10 4LH, UK, pers. comm.). However, this is the first finding of this organism affecting V. myrtillus in the UK.