PHAGE SENSITIVITY IN RELATION TO PATHOGENICITY AND VIRULENCE OF THE COTTON BACTERIAL-BLIGHT PATHOGEN OF SUDAN

At least two pathotypes of Xanthomonas campestris pv. malvacearum are now known to exist in Sudan. The pre-Barakat (race 1) and post-Barakat (race 2) pathogens have been shown to exhibit different host specific ity. The former is pathogenic and highly aggressive on only cultivars with no resistance genes or with the B-2 and/or B-3 resistance factors , while the latter can infect the Bg cultivars also. Race 2 in Sudan, which was previously reported to infect all the standard differentials , produced milder angular leaf spot symptoms and occasionally restrict ed vein infection. Moreover, it exhibited reduced growth in planta com pared with race 1. Bacteriophage studies revealed that the two races a re quite distinct in their phage sensitivity. Race 1 can be lysed by o nly three, or rarely four, of the six phages used for typing, while ra ce 2 is sensitive to all of them. The present study suggests that phag e 7 may be the type-determining phage for race 2. Race 2 strain mutant s resistant to phage 3 or 4 were found to be sensitive to phage 7 and pathogenic to both Acala and Barakat, although showing marked attenuat ion of virulence. However, mutants resistant to phage 2 or 7 were inse nsitive to all the phages and although they retained their pathogenici ty to Acala, they either lost the ability to infect Barakat or produce d a hypersensitive reaction. The resistance of all mutants was found t o be due to failure to adsorb the homologous phage, indicating a chang e in the cell wall. The association of this with the attenuation of vi rulence suggests that bacterial wall components may function as virule nce determinants in Xanthomonas campestris pv. malvacearum.