Emerging strains of Ralstonia solanacearum in the French West indies Raise new challenges to tomato breeders

Bacterial wilt (BW) caused by Ralstonia solanacearum is a major biotic constraint to tomato crops in the tropics. The development of universally resistant tomato cultivars has always been hampered by the unusual genetic and phenotypic plasticity of the pathogen. Since 1999 in Martinique, the bacterial wilt situation was dramatically changed after the emergence of strains assigned to a brand new genotype, phylotype IIB/sequevar4NPB (phII/4NPB) which displayed an enlarged host range compared to strains previously reported. These emerging strains have rapidly spread to the main tomato crop areas, causing severe yield losses. The virulence/aggressiveness features of these emerging strains were assessed on a range of 11 tomato lines, including the major resistance sources. Historical strains of phylotype I and phylotype IIA (broad-host range) were used as references. Strains phII/4NPB were more aggressive and virulent compared to the other strains: they wilted the susceptible lines significantly faster, and were able to wilt resistant genitors like Hawaii7996 and CRA66 whereas other strains could not. The field resistance/ tolerance features of tomato commercial varieties and resistant genitors were assessed in two separate trials on a nursery infested by phylotype I strains (2004) and phII/4NPB strains (2006). The wilt indices were higher in the phII/4NPB nursery. Performances of the cultivars were different between the trials, particularly in the case of Craïbo and HW7996. This study underlines the necessity to survey and consider the broad diversity of the R. solanacearum species complex in the development of BWresistant tomato cultivars. It is suspected that phII/4NPB strains may also concern other countries in this part of the world as this genetic group was detected in different collections, in Brazil and Trinidad. (Résumé d'auteur