ASSESSMENT BY RFLP ANALYSIS OF THE POPULATION STRUCTURE OF XANTHOMONAS PATHOVAR MANGIFERAEINDICAE WITHIN A SINGLE FIELD CONTAINING TWO HOST SPECIES
L GAGNEVIN and O PRUVOST
Laboratoire de Phytopathologie, CIRAD-FLHOR, 97455 Saint-Pierre, Ile de la Réunion, France
Background and objectives
The purpose of this study was to evaluate the role of the pepper tree strains in the epidemiology of the disease on mango by systematically collecting and typing strains from an infected mango orchard and from a nearby infected pepper tree windbreak hedgerow.
Materials and methods
Results and conclusions
Avr haplotypes of strains found on mango and pepper trees are always distinct and constitute separate branches of the neighbour-joining tree. This implies that even if bacterial strains were transported from the pepper tree hedgerow to the mango trees, they did not cause disease on this plant. Several hypotheses can be invoked to explain this: either the lesions on the pepper tree do not produce sufficient amounts of inoculum to infect significantly the mangos, the survival of the pepper tree strains is low during the transport and the epiphytic phases of their life cycle, or the pepper strains can not cause detectable levels of disease or replicate on mango. A combination of these different factors (and others) could also explain the observed absence of pepper tree-type strains on mango.
These results show that, even for strains of a pathovar that cannot be differentiated by means other than genomic studies, there are pathogenicity and epidemiological differences that are probably responsible for the distribution of different members of this pathovar on different hosts. This also raises questions about the relatedness of the haplotypes found on mango and pepper tree. Considering that strains found on the same host are genetically related (as suggested by the neighbour-joining tree) and that some groups of pepper tree-originating strains are close to some mango strains, it is likely that this host differentiation originates from subtle genetic changes revealed by RFLP. This is even more striking considering that the probe used to assess these differences is homologous to avr genes, which are highly involved in host range and probably also in pathogenicity.