First report of leaf spot caused by Xanthomonas campestris on poinsettia in China

In October 2003, a necrotic symptom of new aetiology was observed on poinsettia leaves (Euphorbia pulcherrima cv. Fu-xing) at a flower nursery in the Zhejiang province of China. Tiny, pinpoint, water-soaked lesions scattered across the leaf surface, rapidly enlarged becoming yellow-to-tan. Spots often coalesced to form large, necrotic areas surrounded by a yellow halo, which at times extended into the lateral veins causing a blighted appearance (Fig. 1). No spots were observed on the coloured bracts. Bacterial exudates often formed on lesions, especially on the lower leaf surface (Fig. 2).

A bacterium was isolated from symptomatic leaves on nutrient agar (NA). Infiltration of tobacco leaves with the bacterial suspension resulted in typical hypersensitivity reactions within 24h. On YDC medium, colonies were smooth, round, convex and yellow after 48h. Classical bacteriological tests (Schaad et al., 2001) showed that the bacterium was a Gram negative, single rod, non-motile and positive for catalase and oxidase. It was obligately aerobic; producing levan from sucrose, mucoid growth on NA supplemented with 5% glucose and growing in nutrient broth containing 5% NaCl. It produced acid from glucose, mannose, glycerol, melibiose, trehalose, cellobiose and fructose, but not from arabinose. The maximum growth temperature was 39°C. The bacterium was identified as X.campestris by Biolog and fatty acid methyl ester (FAME) analysis with 0.49 and 0.62 similarity indices when profiles were compared with the Biolog identification database GN4.01 and the MIDI identification database TSBA50, respectively. Leaf inoculation of disease-free poinsettia with a suspension of the isolated bacterium (10⁸ CFU per ml), induced leaf spot symptoms consistent with field observations after 2 weeks.

The bacterium was identified as X.campestris pv. poinsettiicola (Xcp) following the more traditional classification of xanthomonads. Xcp strains from India, America and New Zealand have been re-classified into three separate species (Vauterin et al., 1995). However, available data either by Biolog or FAME databases for the Chinese strain were not sufficient for it to be allocated to the newly proposed taxa. The bacterium is deposited at the BCCMTM , Belgium as R22578. This is the first report of Xcp causing bacterial leaf spot on poinsettia in China.