S RESTREPO^1,2, C VELEZ² and V VERDIER^1-2

¹Centro Internacional de Agricultura Tropical (CIAT), Biotechnology Research Unit, AA 6713, Cali, Colombia, and ²Institut Français de Recherche Scientifique pour le développement en Coopération (ORSTOM), BP5045, 35032 Montpellier, France

Background and objectives

Cassava Bacterial Blight (CBB) is a widespread disease affecting cassava (Manihot esculenta Crantz) crop. It is caused by Xanthomonas axonopodis pv. manihotis (Xam), that can induce a wide variety of symptoms including angular leaf spots, blight, gum exudation, wilting, stem cankers and die-back. To guide the selection and deployment of resistant genotypes, it is essential to characterize the pathogen population structure. Genetic diversity of Xam was mainly characterized by RFLP analyses using a Xam plasmidic probe (pthB) related to the avr-pth gene family. The RFLP/pthB analysis has shown to be more discriminative than RFLP with genomic Xam probes or ribotyping [1]. The distribution of pathogen diversity among different agroecosystems in Colombia was conducted during 1995, showing that a geographical differentiation of pathogen populations exists [1]. Specific RFLP haplotypes were characterized in each of the ecological zones studied [1]. Our goal was to continue the analysis of Xam population structures in Colombia by examining the variation and distribution of the pathogen in fields located in each of the ecological zones previously studied.

Materials and Methods
In 1997, three ecological zones were visited and eight fields were sampled intensively. A large collection of Xam strains was isolated. DNA polymorphism was assessed by RFLP analyses and REP-PCR. Genetic diversity and phylogenetic relationships among strains were determined as described by Vera Cruz et al. [2]. All the strains were tested for virulence on the cassava susceptible variety MCOL1522.

Results and conclusions
The results of genome analysis indicated that the bacterial blight pathogen is variable and that new forms developed in specific ecological zones. New haplotypes were detected that are different from the types described before. In the same ecological zone, genetic diversity varied greatly among fields. The highest level of genetic diversity among Xam strains was found within a field located in the ecological zone 2-5. In that field, a broad range of cassava genotypes was cultivated. Our results suggest a role of the host in causing differentiation of the pathogen.