The use of biocontrol plants to manage bacterial wilt of tomato in the tropics

Bacterial wilt (BW), caused by Ralstonia solanacearum, is a major disease affecting tomato crops worldwide. In Martinique (French West Indies), the situation concerning BW has changed dramatically since 1999, with the emergence and rapid spread throughout the island of a new genotype of R. solanacearum, phylotype IIB/4NPB. This genotype wilts tomato cultivars previously considered as resistant and cannot be controlled by any conventional method. The aim of this study was to contribute to BW management via the introduction of selected plant diversity in existing vegetable cropping systems. We investigated the use of previous crops with biocontrol capacities as sanitizing crops during the cultivation and/or decomposition stage. The ability of the emergent population of R. solanacearum to persist in planta and in the rhizosphere of Brassicaceae, Asteraceae and Fabaceae grown as previous crops was first assessed under controlled conditions. Then, the most active phase of plants with biocontrol capacities was determined in the greenhouse on a naturally infested soil. Finally, the potential of three Fabaceae to control BW of tomato was evaluated under field conditions. Results under controlled conditions showed that all previous crops tested were symptomless carriers of R. solanacearum, and that the density of R. solanacearum in the rhizosphere differed between cultivars within the same species and between species within the same genus. Under greenhouse conditions, Crotalaria juncea and Crotalaria spectabilis grown as previous crops significantly decreased BW incidence on tomato, and the disease incidence was lower after the growing phase of Crotalaria spp. than after the soil incorporation phase. In the field, C. juncea and C. spectabilis decreased BW incidence on tomato by 71 and 58%, respectively. These results suggest that C. juncea and C. spectabilis could be used as sanitizing plants to help BW control in ecological management strategies, although without drastic suppression of R. solanacearum populations