DETECTION OF THE CASSAVA BACTERIAL-BLIGHT PATHOGEN, XANTHOMONAS-AXONOPODIS PV. MANIHOTIS, BY POLYMERASE CHAIN-REACTION

Cassava bacterial blight, caused by Xanthomonas axonopodis pv. manihot is, is of significant concern wherever cassava is grown. The movement of infected, asymptomatic stems is a major means of pathogen dispersal . A reliable and sensitive diagnostic procedure is necessary for the s afe movement of cassava planting material. We used a cloned and sequen ced pathogenicity gene of X. axonopodis pv. manihotis to develop a pol ymerase chain reaction (PCR) test for this pathogen. A set of primers directed the amplification of an 898-bp fragment in all 107 pathogenic strains of X. axonopodis pv. manihotis tested. PCR products were not observed when genomic DNA was tested for 27 strains of other xanthomon ads, for saprophytic bacteria, or for five nonpathogenic strains of X. axonopodis pv. manihotis. The primers worked well for pathogen detect ion in direct PCR assays of X axonopodis pv. manihotis colonies grown on liquid medium and in PCR assays of extracts from leaf and stem lesi ons. The minimum number of cells that could be detected from cassava s tem and leaf lesions was 3 x 10(2) to 10(4) CFU/ml. The PCR assays pro ved to be relatively sensitive and could become very useful in detecti ng the pathogen in cassava planting material.