Genetic diversity and variability in pathogenicity of xanthomonads pathogenic to sugarcane in Réunion Island

Several xanthomonads can cause diseases in sugarcane and they often induce similar symptoms (necroses, stripes, whitening,...): Xanthomonas albilineans, causal agent of leaf scald; X. axonopodis pv. vasculorum, causal agent of gumming; X. vasicola pv. holcicola, causal agent of sorghum leaf streak; and Xanthomonas sp., causal agent of false red stripe in Brazil. Leaf scald and gumming are vascular diseases that occur in many sugarcane growing locations including Reunion Island. Use of resistant cultivars is the most effective means of control and, therefore, susceptible cultivars should be eliminated in breeding programs. Efficient screening for resistance should be based on a good knowledge of diversity and variability of these pathogens. Avr (avirulence) and hrp (hypersensitive response and pathogenicity) genes have been identified within many pathogenic bacteria. These genes are involved in virulence but also in agressiveness or fitness of bacterial strains. In this study, genetic diversity of xanthomonads pathogenic to sugarcane was determined by RFLP (restriction fragment length polymorphism) using i/ the pXV9 probe (cluster hrp) from Xanthomonas axonopodis pv. vesicatoria and the product of EcoR1 digestion of the genomic DNA of the four Xanthomonas species from sugarcane, and ii/ the pBSavrXa10 probe of Xanthomonas oryzae pv. oryzae and the product of BamH1 digestion of the genomic DNA of the same bacterial species. The pXV9 probe hybridized with the DNA of all xanthomonads pathogenic to sugarcane with the exception of X. albilineans, and each species showed unique RFLP profiles. The avrXa 10 probe did not hybridize with the DNA of X. albilineans and X. vasicola pv. holcicola. Eleven haplotypes were identified among the 41 strains (30 strains from Reunion Island and 11 strains from other locations) of X. axonopodis pv. vasculorum. These 11 haplotypes were clustered in three different groups by a dissimilarity analysis. The first group included six haplotypes belonging to 22 strains isolated from sugarcane in Reunion Island. The second group was formed by four haplotypes and gathered eight strains from Reunion Island and nine strains from Mauritius and Australia isolated from sugarcane but also from other plants (Thysanolaena maxima, Dictyosperma album, Roystonea regia). The third group comprised a unique haplotype representing two strains from Thysanolaena maxima in Mauritius. Pathogenicity in sugarcane of six strains of X. axonopodis pv. vasculorum from Reunion Island was determined in a field trial. Progress of symptoms varied with the strain. No relationship between variability of pathogenicity and genetic diversity revealed by the avrXa10 probe was, however, observed. (Texte integral)