Etiology Of Poinsettia Leaf Spot And Its Antagonistic Bacteria In China

As the economic development, more and more people prefer flowers, especially Euphorbia pulcherrima. Over 5 millions pots are produced for sale in China every year, however, poinsettia was observed to be attacked by two kinds of plant pathogens causing leaf spot in flower nursery and resulted in great losses in Zhejiang Province and Shanghai city in October 2003. Based on pathogenicity test, morphological and cultural characterization as well as physiological and biochemical properties, the pathogens isolated from the diseased leaf with different symptoms were identified as Botrytis cinerea and Xanthomonas campestris pv. poinsettiicola, respectively. It is first reported that bacterial leaf spot of poinsettia occurred in China.Characterization of the causative xanthomonad was performed by a polyphasic approach. Biolog breathprints of 3 bacterial strains isolated from China did not allow formal conclusions on species allocation within the genus Xanthomonas. Although comparisons of bacterial fatty acid profiles revealed "closet match" with Xanthomonas axonopodis, the similarity indices of these matches were inadequate for species. Analysis of 16S rRNA gene sequence similarities indicated that the causal agent belongs to the campestris core in the genus Xanthomonas (which is more precisely in rRNA cluster 1). The whole rRNA sequence of strain R-22578 has been submitted to EMBL databases (Accession No.AJ811695).The 16S-23S rDNA internally transcribed spacer (ITS) sequences were determined to investigate whether the 16S-23S ITS region is useful for differentiating the 3 Xanthomonas strains causing bacterial leaf spot of poinsettia in China from the type strains of known Xanthomonas species. In the phylogenetic analysis based on these sequences, three strains from China and 6 strains of X. axonopodis clustered within a group and well separated from other species of Xanthomonas. It is therefore considered that the leaf spot Xanthomonas strains from China should be classified as X. axonopodis.The 3 strains of Xanthomonas from China were differentiated from the pathotype strain LMG849 of X. axonopodis pv. poinsettiicola causing bacterial leaf spot of poinsettia by comparison of pathogenicity, substrate utilization and genomic fingerprints generated by using the BOX-AlR primer. Based on ITS sequences, primer (BSLP-R, BSLP-F) was designed for detection of the causal agent, however the specificity of primer is still to improve.Seventeen strains of fluorescent Pseudomonas and 4 strains of gram-positive bacteria were isolated and screened for inhibition of the causal agent of poinsettia in vitro. The antagonistic effect of these strains used as seed treatment or soil drench on root knot nematode (Meloidogyne javanica) that caused great losses on poinsettiia and many other greenhouse plants were evaluated on greenhouse in Belgium. Three strains (B7,5a-4 and Y3-1) significantly reduced the nematode populations in the rhizosphere and enhanced the growth of mungbean plants over the controls in the presence or absence of Rhizoctonia solani SX-6, however, the application of bacterial mixtures did not show greater effect for disease control and plant promotion than that of single strain.The inhibition effect of 17 antagonistic bacteria against Fusarium oxysporum f.sp. melonis, the causalorganism of Fusarium wilt of melon, were determined by counter-plated method. Seven of them were highly effective in suppressing the F oxysporum f.sp. melonis and 5 other pathogenic fungi in vitro within 7 days. The culture filtrates of antagonistic bacteria suppressed the mycelial growth and conidial germination of F oxysporum f.sp. melonis. The inhibition effect wasn't reduced greatly when culture filtrates of antagonistic bacteria were treated with proteinaseK and protein extraction with 70% (NH₄)₂SO₄ exhibited little antagonistic activity, which showed that antibiotics rather than protein might involve in the main mechanism of antagonism. Three of the antagonistic bacteria, which were identificated as Pseudomonas aeruginosa by morphological, physiolog...