CBN-V Video Archives - S8-18
Cassava bacterial blight: recent advances in
the understanding and control of the disease

Restrepo S., Mosquera, G.M., Velez, C.M., Lopez, C.E., Zuluaga, P., Gonzalez, C., Chavez, M., Santaella, M., Suarez, E., Jorge, V., Lopez, A., Pineda, R., Garcia, S., Ojeda, S., Tohme, J., and Verdier, V.

1. CIAT Biotechnology Reasearch Unit, CIAT, A.A. 6713, Cali, Colombia
2. IRD, Montpellier, France
3. University of Los Andes, Bogota, Colombia

        Xanthomonas axonopodis pv. manihotis (Xam) is an important bacterial pathogen of cassava causing the bacterial blight.  Pathogen population structure was studied between 1995 and 1999 in Colombia using molecular markers (RFLP, AFLP) and virulence assays.  A first study conducted in 1995 and 1996 revealed the geographical differentiation of RFLP haplotypes in ecozones.  Over the 5 year period, haplotype frequencies significantly differ in most of the locations where strains were collected.  Different pathogens were characterized using a set of differential cultivars adapted to each ecozone.  The evolution of pathotypes over time is also discussed.  A molecular genetic map of cassava was recently constructed from a F1 cross of non-inbred parents.  RFLP, AFLP, EST, SSR markers were used to map resistance to bacterial blight in cassava.  The F₁ cross was evaluated with Xam strains under both field and greenhouse conditions.  Nine QTLs (quantitative trait loci), located on linkage groups B, D, L, N, and X, were found to explain the phenotypic variance of the crop's response to Xam  in greenhouse.  Linkage group D has been also found involved in field resistance.  The cDNA-AFLP technique was developed and the amplification of resistance gene analogs (RGAs)n employed as means of elucidating the putative genes involved in the defense response.  For the cDNA-AFLP technique, of ~3600 cDNA fragments screened, 353 fragments were specific to the resistant variety.  Sequence analyses showed significant homology with resistance genes, NPK-1 related proteins, senescence-related proteins and other known proteins involved in disease resistance reactions.  Using degenerate primers we identified twelve different classes of resistance-gene analogs (RGAs) from cassava.  Screening a cassava cDNA library (root and Leaf) with class-specific RGAs probes allowed the identification of sixteen expressed clones.  Sequence analysis of clone L16 confirmed the constitutively expression of a protein that shares characteristics with previously reported resistance genes.  The movement of infected asymptomatic stems is a major means of pathogen dispersal as well as infected seeds.  The success of a cassava-seed certification program depends on the availability of reliable tests to detect the pathogen in vegetative planting materials and true seeds.  We report here the different methods that permitted to detect the pathogen in cassava tissues.

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