First report of a phytoplasma associated with Praxelis witches’ broom in China

Praxelis (Praxelis clematidea) is native to South America (King & Robinson, 1970), but in the last 20 years has become highly invasive in Australia, Southeast Asia and more recently in south China (Waterhouse, 2003). As one of the most highly invasive plants in these regions, it poses a serious threat to their ecosystems. Biological control may offer the most promising sustainable control strategies for exotic weeds and since 2005, disease surveys have been conducted in the hope of identifying potential biocontrol pathogens for Praxelis. Witches’ broom symptoms, suggestive of phytoplasma infection, were observed in Hainan, China. Plants showed small pink-coloured leaves and much proliferation of shoots with shortened internodes. These plants failed to produce seed (Fig. 1).

Healthy and infected Praxelis were collected from Danzhou, Hainan and tested by PCR for phytoplasmas. Nucleic acids were extracted from fresh leaves as described by Parmessuret al.(2002) and used as a template in nested PCR with primers R16mF2/R16mR1 and R16F2/R2 (Leeet al., 1998). DNA from phytoplasma infected periwinkle was used as a positive control. A DNA fragment (1189bp) was amplified from diseased plants but not from known healthy or symptomless plants. PCR products were purified and sequenced, and the sequence deposited in GenBank (accession no. EF061924). Using a multiple alignment program (DNASTAR Version 6.13) the sequence showed a high similarity with members of the 16SrII group of phytoplasmas, and had the highest similarity (98.5%) with phytoplasmas from the 16SrII (Peanut witches'-broom) group (L33765), sub group-A (Leeet al., 1998), including sweet potato witches’ broom (DQ452417) and Chinese pigeon pea witches’ broom (AF028813) (Fig. 2). To our knowledge, this is the first report of a phytoplasma from the peanut witches’ broom group infecting Praxelis.