Eleven strains of Xanthomonas campestris pv. graminis, causal agent of grass bacterial wilt, collected over 20 years, varied significantly (P < 0.001) in their aggressiveness on meadow fescue and Italian rye grass. Aggressiveness of strains isolated since 1993 was not higher than strains isolated from 1975 to 1984. Significant interactions (P < 0.001) between X. campestris pv. graminis strains and cultivars occurred for meadow fescue but not for Italian rye grass (P =0. 934). Inoculation with a single, highly aggressive strain for breeding of meadow fescue proved to be useful to increase the resistance to bacterial wilt, but improvement became stagnant after an initial strong increase in resistance level. Furthermore, a possible breakdown of the resistance of meadow fescue cultivars when grown in exotic environments might be possible. Therefore, the use of a strain mixture for breeding of meadow fescue is recommended.