THE IMPORTANCE OF RHIZOBACTERIAL MOBILITY IN BIOCONTROL OF BACTERIAL WILT OF TOMATO

Studies were done to determine if the mobility of rhizobacteria in the rhizosphere enhances their ability to control bacterial wilt of tomato caused by Pseudomonas solanacearum. The term is used in this study to denote the ability of a bacterium to follow developing roots in the absence of percolating water. Of 65 rhizobacterial isolates of Bacillus sp., P. fluorescens, and P. putida, collected from the rhizosphere of field-grown tomato plants. 10 different isolates that exhibited mobility in the rhizosphere (group 1) and 10 others that did not (group 2) were selected for a comparative study. Members of the two groups were tested four times for mobility, biocontrol activity (the ability to reduce P. solanacearum-induced mortality), and growth in the rhizosphere. All isolates in group 1, but none in group 2, showed mobility in repeated experiments and the groups were significantly different (P = 0.0001) from each other with respect to the average minimum distance below the crown where each isolate was recovered. Biocontrol activity was significantly greater in group 1 than in group 2. Group 1 isolates reduced percent plant mortality significantly (P = 0.005), compared to untreated controls while group 2 isolates did not, and percent plant mortality in group 1 was significantly different from group 2 (P = 0.0006). All isolates in group 1 and only 50% of isolates in group 2 grew in the rhizosphere and the difference between the groups was significant (P = 0.0001). Mobility was negatively correlated with percent plant mortality (P = 0.0001) and positively correlated with the growth in the rhizosphere (P = 0.006). These results indicate that mobility may be an important trait for certain rhizobacteria in controling disease.