First report of bacterial leaf spot of poinsettia caused by Xanthomonas axonopodis pv. poinsettiicola in Taiwan

Leaf spot symptoms on poinsettia (Euphorbia pulcherrima) were observed during November 2005 on nurseries in the Dongshih Township, Taichung County, Taiwan. The symptoms first appeared on the leaves as small spots, which quickly turned brown and were surrounded by pale yellow haloes. The brown spots and haloes enlarged rapidly and coalesced into irregular, yellow or brown, dry, dead areas on the leaf.

Isolations from diseased leaves consistently yielded bacterial colonies that were yellow and glistening on nutrient and potato dextrose agar media. Five representative isolates were chosen for further characterisation. All isolates were Gram-negative rods, aerobic and produced yellow, non-water soluble xanthomonadin pigments identified by TLC (Schaad et al., 2001). The isolates were positive for catalase and b-galactosidase, but negative for oxidase, nitrate reductase, urease, arginine dihydrolase and tryptophanase (indole production). They hydrolysed starch, gelatine and aesculin, but did not metabolise quinate (tested in SQ medium; Lee et al., 1992). In Hayward's medium, acids were produced from arabinose, cellobiose, glucose, maltose, mannose and sucrose, but not from dulcitol, lactose, mannitol and sorbitol. Thus, the bacterium was presumptively identified as a Xanthomonas spp. Biologâ„¢ analysis identified the bacterium as X. campestris when GN2 plate profiles were compared with the GN601 identification database. An almost complete 16S rDNA sequence from isolate XAP1 (1,485 bp; Acc. No. DQ414814) was compared with 16S rDNA sequences in GenBank and was identical (99-100%) to those of several xanthomonads, including: X. arboricola (Y10757), X. axonopodis pv. citri (AE012082), X. campestris pv. campestris (AE012540) and X. campestris pv. poinsettiicola (AJ811695).

To fulfil Koch's postulates, bacterial suspensions (108 CFU per ml) were injected into the leaves of four poinsettia plants. Inoculated plants were kept in a growth chamber at 28°C. Typical symptoms were observed in 6-10 days in all inoculated plants and were identical to those observed on the nurseries. Control plants, inoculated with sterile distilled water, showed no symptoms. The bacterium was readily re-isolated from diseased leaves. Euphorbia milii and Codiacum variegatum (both Euphorbiaceae) were also inoculated; symptoms appeared on E. milii but not on C. variegatum.

Bacterial leaf spot of poinsettia was first reported in India and the pathogen was identified as X. campestris pv. poinsettiicola (Patel et al., 1951). X. campestris pv. poinsettiicola strains were reclassified into three separate species: X. arboricola pv. poinsettiicola, X. axonopodis pv. Poinsettiicola and X. codiaei (Vauterin et al., 1995). X. arboricola can metabolise quinate (Lee et al., 1992) and X. codiaei causes disease on C. variegatum, but the bacterium isolated in Taiwan did not have either capacities and should be allocated to X. axonopodis pv. poinsettiicola. This is the first report of this bacterium on poinsettia in Taiwan.