POPULATION-DYNAMICS OF XANTHOMONAS-ALBILINEANS IN SUGARCANE PLANTS AS DETERMINED WITH AN ANTIBIOTIC-RESISTANT MUTANT

Characteristics of a spontaneous mutant of Xanthomonas albilineans (GPE 5SR) resistant to streptomycin and rifampicin were not different from those of the wild parental strain (GPE 5) in cultural, biochemical, and immunological properties. The mutant strain was at least as virulent as the parental wild strain and was used as an epidemiological tool for the study of the population dynamics of X. albilineans. Populations of X. albilineans GPE 5SR and symptom expression of leaf scald were monitored in six sugarcane clones differing in resistance to the disease. The pathogen invaded leaves, stalks, and roots. The apex of the stalk including the apical meristem was invaded. Symptom expression and population size in leaves and shoot apices were correlated under field conditions. In contrast, the pathogen populations in the older portion of stalks increased initially after inoculation and then remained stable for both studied clones, whatever the progress of the disease. Sugarcane inoculated with X. albilineans and maintained under greenhouse conditions exhibited few or no symptoms but was colonized by the pathogen. Pathogen populations were highest in susceptible clones. Thus, high population densities of X. albilineans in sugarcane are necessary but not always associated with development of disease symptoms. Nevertheless, the correlation between colonization of plants and leaf scald resistance may be useful for screening sugarcane clones for resistance to the disease.