Abstract
The colony type variants of Xanthomonas campestris pv. oryzae derived from wild type during subculture on PSA medium were greatly different in virulence one another and most of them were attenuated as compared to wild type. Virulence of these variants was closely related with colony types, serological properties, autoagglutination in 1% CaCl2 solution, phage sensitivity and ability of slime polysaccharide production. In general, the colony type variants Lw (large, smooth, waxy yellow) and Lh (Large, dark yellow, with halo) of serovar A produced greater amount of slime polysaccharide and were more virulent than the variants St (small, transparent) and Sm (small, mucoid) of serovar B-II. The variants Ld (large, dark yellow) and Lh of serovar B-I were intermediate between serovar A and B-II in virulence and polysaccharide production. Avirulent and weakly virulent variants of serovar B-I and B-II agglutinated remarkably in 1% CaCl2 solution while wild type and virulent variants did not. No correlation was observed between their bacteriological properties and pathogenic specialization. Weakly virulent and avirulent variants interfered with wild type to suppress symptom expression. The decrease of virulence during subculture of X. campestris pv. oryzae was considered to be mainly caused by the population increase of weakly virulent and avirulent variants and partially caused by interference of these variants to wild type in symptom expression.
