Increased susceptibility to bacterial wilt in tomatoes by nematode galling and the role of the Mi gene in resistance to nematodes and bacterial wilt

The soil-borne bacterial pathogen Ralstonia solanacearum commonly coexists with polyspecific nematode populations in tropical and subtropical areas. The wounding of roots by nematodes is usually invoked to explain the correlation between nematode infection and bacterial wilt, since this wounding increases the number of sites for bacterial entry. 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 bacterial isolate GMI 8217 and two different sedentary plant parasitic nematodes, the endoparasitic root-knot nematode, Meloidogyne incognita, and semi-endoparasitic reniform nematode, 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 root-knot nematode. At high temperatures (27-32°C), the root-knot nematode greatly increased wilt severity in susceptible Floradel and resistant Caraïbo, but the reniform nematode had no such effect regardless of temperature x cultivar combination. This showed that infection of tomato roots by root-knot nematodes reduced genetic resistance to bacterial wilt. The effects of combining bacterial wilt resistance with the Mi gene for resistance to root-knot nematode (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