First report of fruit rot caused by Phytophthora syringae on Pyracantha in the UK

Pyracantha (firethorn) is grown mainly for its display of red, orange or yellow fruit in the autumn and winter and is also used as a hedge plant. In 2001 and 2002, two samples of Pyracantha berries (both orange varieties) from Hampshire (RHS/10715/01) and from North Yorkshire (RHS/13308/02), showing signs of rotting were received at the advisory service of the Royal Horticultural Society. The symptoms observed were brown/black spots that enlarged, with eventually the whole berry turning black and shriveled. Whole clusters of berries were affected but the symptoms were only observed on berries produced on the lower branches of the Pyracantha shrubs.

A Phytophthora species was isolated following the baiting of the affected berries in apple and plating on P10VP selective medium (Erwin & Ribeiro, 1996). Sporangia were produced and zoospores released in pond water, following floating of the apple baits. Phytophthora syringae was identified using morphological characteristics and confirmed by DNA sequencing. Sporangia formed sympodially, were semi-papillate, ovoid, non-caducous with average dimensions of 41 x 25 µm. Hyphal swellings were produced abundantly in chains. The fungus was homothallic and the antheridia, where apparent, were paragynous. Plerotic oogonia (average 28 mm diameter) were produced readily on carrot agar (CA) medium (Ribeiro, 1978). On 2-week-old CA plates, the antheridium was obscured by a knot of hyphae. The identification of the isolate RHS/10715/01 was confirmed by direct sequencing of the ITS region using primers ITS 6 and ITS 4, as described by Cooke et al. (2000). The nucleotide sequence obtained (Accession No. AY611644) was almost identical (with only 2 base substitutions) to Phytophthora syringae strains IMI296829 (AF266803), CBS132.23 (AF380146) and PDR115773A (AY423302).

Koch's postulates were fulfilled by floating berries from P. coccinea x P. rogersiana (orange fruiting variety) in a suspension of zoospores (2-3 x 106spores per mL) at room temperature. The berries were wounded before infection, by producing a slit with a scalpel into the fruits. As a control, wounded berries were also floated in filtered pond water. Black spots started appearing on the fruits at 10 days post-infection and the fungus was re-isolated from the infected tissues by baiting with apples. A wound was necessary for successful infection. The control berries did not develop any symptoms.

Phytophthora syringae has a fairly wide host range and is known to cause fruit rot on Citrus, Pyrus and Malus (Erwin & Ribeiro, 1996). This is however the first report of this disease on Pyracantha in the UK and also worldwide.