Increased susceptibility to bacterial wilt in tomatoes by nematode galling and the role of the Mi gene in resistance to nematodes and bacterial wilt.
Bacterial wilt development on tomato was investigated in a controlled environment on the susceptible tomato cultivar Floradel and the polygenically wilt-resistant cultivar Caraïbo. The Ralstonia solanacearum isolate GMI 8217 and Meloidogyne incognita and Rotylenchulus reniformis, were cross-infected. At low temperatures (22-27°C), the bacterium GMI 8217 was slightly pathogenic on all tomato lines, except on Floradel coinfected by M. incognita. At high temperatures (27-32°C), M. incognita greatly increased wilt severity in susceptible Floradel and resistant Caraïbo, but R. reniformis had no such effect regardless of temperature × cultivar combination. This showed that infection of tomato roots by M. incognita reduced genetic resistance to bacterial wilt. The effects of combining bacterial wilt resistance with the Mi gene for resistance to M. incognita (Mi, resistant; Mi+, susceptible) was investigated using the near-isogenic lines Caraïbo (Mi+/Mi+)/Carmido (Mi/Mi) and CRA 66 (Mi+/Mi+)/Cranita (Mi/Mi), which differ by the size of the segment of L. peruvianum DNA carrying the Mi gene. The presence of the Mi gene was associated with a marked decrease in bacterial wilt resistance. It is suggested that at least one gene governing part of the bacterial wilt resistance is closely linked or allelic to the Mi gene in the tomato lines used in this experiment.