A streptomycin- and rifampicin-resistant mutant of Xanthomonas albilineans was used to study symptom expression of leaf scald disease (LSD) and colonization of sugarcane (Saccharum spp) and its wild relatives by this bacterial pathogen. A total of 40 sugarcane cultivars and 15 clones from the S accharum complex that differed in resistance to LSD were inoculated by a decapitation technique in both field and greenhouse experiments. In the plant crop, disease severity varied between 0 for the most resistant genotypes and 100 for the most susceptible ones. Resistance to LSD was characterized by limited colonization of the host plant by X. albilineans. Although almost all genotypes were colonized by the pathogen, the greatest bacterial population densities found in the susceptible cultivars. These was a high correlation between disease severity and pathogen population in the apex. Several genotypes exhibed no or slight symptoms even though they were highly colonized in the upper and /or basal nodes of stalk. Two mecanisms, therefore, may play an important role in resistance to LSD : resistance to colonization of the apex, which is characterized by absence of symptoms, and resistance to colonization of the upper and lower parts of stalk. In contrast, disease severity and pathogen population densities in the first ratoon crop in the field were nil or very low in the stalks, except for the highly susceptible cv. CP68-1026. Sugarcane ratoons therefore, may recover from the disease after plant cane infection. Nevertheless, because low levels of the pathogen were still detected in some stalks, it is possible that LSD could develop from latent infections if favorable environmental conditions occur.