The variability of Ralstonia solanacearum isolates obtained from tomato (Lycopersicon esculentum) plants in the Amazonas State was studied for pathogen aggressiveness, bacteriocin sensitivity and for biochemical characteristics. Three experiments were established in areas naturally infested with R. solanacearum. These were: (i) in a non-flooded area of a field in 1998, (ii) in an periodically-flooded area, in 1998, and (iii) in the same periodically-flooded area in 2000. In each experiment there were 200 resistant tomato plants (cv. Yoshimatsu) and 200 susceptible (cv. Santa Cruz Kada) planted by alternating resistant and susceptible cultivars. From the 267 isolates obtained from plants exhibiting wilt symptoms, 67.8% belonged to biovar 1 and 32.2% to biovar 3. In the non-flooded soil, 80.4% of he isolates collected were found to be biovar 1, whereas in fields periodically-flooded, the percentage of biovar 1 isolates was 37.4 and 87.8% for the 1998 and 2000 experiments, respectively. Based on bacteriocin sensitivity classification, isolates were divided into seven groups and two of these groups accounted for 80.5% of the isolates. The aggressiveness of selected isolates was evaluated in tomato, eggplant (Solanum melongena) and sweet pepper (Capsicum annuum) plants. Biovar 1 isolates were more aggressive on tomato than biovar 3 isolates. Biovar 3, however, was more aggressive on sweet pepper and eggplant than biovar 1. In fields subject to periodic flooding, cv. Yoshimatsu had 50.5% mortality versus 22.5% mortality in the non-flooded field. This indicates the importance of this environmental factor on disease occurrence.
