Biocontrol of Ralstonia solanacearum by Treatment with Lytic Bacteriophages

Ralstonia solanacearum is a Gram-negative bacterium and the causative agent of bacterial wilt in many important crops. We treated R. solanacearum with three lytic phages: phi RSA1, phi RSB1, and phi RSL1. Infection with phi RSA1 and phi RSB1, either alone or in combination with the other phages, resulted in a rapid decrease in the host bacterial cell density. Cells that were resistant to infection by these phages became evident approximately 30 h after phage addition to the culture. On the other hand, cells infected solely with phi RSL1 in a batch culture were maintained at a lower cell density (1/3 of control) over a long period. Pretreatment of tomato seedlings with phi RSL1 drastically limited penetration, growth, and movement of root-inoculated bacterial cells. All phi RSL1-treated tomato plants showed no symptoms of wilting during the experimental period, whereas all untreated plants had wilted by 18 days postinfection. phi RSL1 was shown to be relatively stable in soil, especially at higher temperatures (37 to 50 degrees C). Active phi RSL1 particles were recovered from the roots of treated plants and from soil 4 months postinfection. Based on these observations, we propose an alternative biocontrol method using a unique phage, such as phi RSL1, instead of a phage cocktail with highly virulent phages. Using this method, phi RSL1 killed some but not all bacterial cells. The coexistence of bacterial cells and the phage resulted in effective prevention of wilting.