First identification of ‘Candidatus Phytoplasma aurantifolia’ infecting Fallopia japonica in the United Kingdom

Japanese knotweed (Fallopia japonica), family Polygonaceae, is a tall, invasive, perennial weed, first introduced into the UK in the 1850’s. It has since spread to occupy more than half of the 10 km squares used to map plant distribution in the British Isles. The plant causes problems in both rural and urban habitats due to its vigorous growth and ability to form dense thickets. In urban environments its growth can damage buildings and push up tarmac.

In July 2008, plants of F. japonica growing along a boundary fence in Egham were seen with severe stunting and witches’ brooms. Symptoms were most pronounced in the emerging shoots, which displayed proliferation, shortened internodes and small leaves (Figs 1 & 2). Infected plants also exhibited a pronounced silver flecking on the leaves, which superficially resembled thrip-damage (Fig 3). The knotweed stand occupied an area of approximately 35 m² and 50 of the 88 plants (56.8%) displayed obvious symptoms.

Leaves from four plants with symptoms and one healthy-looking plant were collected and the DNA extracted and indexed in a nested PCR assay with universal primers P1m (Hren et al., 2007) /P7 (Gundersen & Lee, 1996) and fU5/rU3 (Lorenz et al., 1995). PCR products of expected size (~880 bp) were obtained from all plants displaying symptoms, but not from the healthy-looking plant. Nested PCR products of a representative plant were purified, cloned (pGEM-T Easy Vector, Promega), and sequenced in both directions using M13 sequencing primers (www.dnaseq.co.uk). The 16S rDNA sequence was compared with those held in GenBank using BLAST. The closest matches (99%) were to phytoplasma members of group 16SrII, ‘Candidatus Phytoplasma aurantifolia’, including those associated with crotalaria witches’ broom (EU650181, EF656454) and sweet potato little leaf (EU170353). The new phytoplasma sequence was deposited in GenBank (FJ008924).

The severe stunting of Japanese knotweed (Fig. 4) has important implications for its competitive ability and it was noted that stinging nettles (Urtica dioica) were overgrowing many of the stunted plants. We believe this is the first record of a phytoplasma infecting F. japonica,and the first identification of a phytoplasma from group 16SrII in the UK.