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

The soil-borne bacterial pathogen Ralstonia solanncearum commonly coexists with polyspecific nematode populations in tropical and subtropical areas. T he wounding of roots by nematodes is usually invoked to explain the correla tion between nematode infection and bacterial wilt, since this wounding inc reases the number of sites for bacterial entry. Bacterial wilt development on tomato was investigated in a controlled environment on the susceptible t omato cultivar Floradel and the polygenically wilt-resistant cultivar Carai bo. The bacterial isolate GMI 8217 and two different sedentary plant parasi tic nematodes, the endoparasitic root-knot nematode, Meloidogyne incognita, and semi-endoparasitic reniform nematode, Rotylenchulus reniformis, were c ross-infected. At low temperatures (22-27 degrees C), the bacterium GMI 821 7 was slightly pathogenic on all tomato lines, except on Floradel coinfecte d by root-knot nematode. At high temperatures (27-32 degrees C), the root-k not nematode greatly increased wilt severity in susceptible Floradel and re sistant Caraibo, but the reniform nematode had no such effect regardless of temperature x cultivar combination. This showed that infection of tomato r oots by root-knot nematodes reduced genetic resistance to bacterial wilt. T he effects of combining bacterial wilt resistance with the Mi gene for resi stance to root-knot nematode (Mi, resistant; Mi(+), susceptible) was invest igated using the near-isogenic lines Caraibo (Mi(+)/Mi(+))/Carmido (Mi/Mi) and CRA66 (Mi(+)/Mi(+))/Cranita (Mi/Mi), which differ by the size of the se gment of L. peruvianum DNA carrying the Mi gene. The presence of the Mi gen e was associated with a marked decrease in bacterial wilt resistance. It is suggested that at least one gene governing part of the bacterial wilt resi stance is closely linked or allelic to the Mi gene in the tomato lines used in this experiment.