Evaluation of bacterial wilt resistance in tomato lines nearly isogenic for the Mi gene for resistance to root-knot

Resistance to bacterial wilt, caused by Ralstonia solanacearum, in tomato l ines CRA 66 and Caraibo is reported to be decreased by root-knot nematode g alling and by introduction of the Mi gene for nematode resistance. The Mi g ene is located on tomato chromosome 6, which also carries a major quantitat ive trait locus (QTL) for resistance to bacterial wilt. Bacterial wilt resi stance was evaluated in FS-progenies derived from two crosses between near- isogenic lines, Caraibo x Carmido and CRA 66 x Cranita, differing for small and large introgressions from Lycopersicon peruvianum that carry the Mi ge ne, respectively. These introgressed regions were mapped using RFLP markers . Plants homozygous Mi(+)/Mi(+) (susceptible to the nematode) and homozygou s Mi/Mi (resistant) for the Mi gene were selected in F2 and used to produce F3 progenies. Parents and F3-lines with Mi/Mi had resistance to bacterial wilt reduced by 30% in Caraibo x Carmido and by 15% in CRA 66 x Cranita. Ca raibo and Carmido were demonstrated to be isolines and the small introgress ion from L. peruvianum resulted in loss of the QTL for bacterial wilt resis tance, which is probably allelic or linked in repulsion to the Mi gene. In contrast, resistance to bacterial wilt segregated in the F3 lines from the cross CRA 66 x Cranita, giving families varying in resistance between the l evels shown by the parents. Consequently, two hypotheses were considered: ( i) after only four backcrosses, the parents were not isolines and the genes for resistance to bacterial wilt from CRA 66 were still segregating, and ( ii) the parents were isolines and variation in resistance to bacterial wilt in F3 was due to recombination events among the large L. peruvianum introg ressed chromosome region from Cranita.