Improving Tolerance to Bacterial Spot of Tomato through Fine Mapping of Two Quantitative Trait Loci

Tomato (Solanum lycopersicum) is one of the most economically important vegetable crops in the United States, and annual production value is estimated over $2.5 billion (2009). Bacterial spot is caused by at least four species of Xanthomonas and is a major disease of field-grown tomatoes in many humid production regions. The disease causes defoliation and reductions in both total and marketable yields. There is no good control measure available for the disease, and host resistance is an attractive goal for disease management. Previous research determined that a quantitative trait locus (QTL) on chromosome 11 confers bacterial spot resistance, while increased susceptibility is associated with the fusarium wilt resistant gene, I-3. In this study, we are using a map-based cloning approach to fine map the chromosome 11 QTL. Approximately 40 semi-isogenic recombinant inbred lines were developed by identifying recombinants for the chromosome 11 introgression in populations derived from the tolerant lines, Fla. 8517, Fla. 8233 and Fla. 8286; and backcrossing these recombinants three times to the highly-susceptible line, Fla. 7946. The introgression spans approximately 35 Mbp, including the centromere. Severe recombination suppression throughout much of the region has limited identification of useful recombinants. A similar approach is being employed to determine if the I-3 association with bacterial spot susceptibility is the result of pleiotropy or linkage with unfavorable alleles. I-3 is located within a 1 Mbp region on chromosome 7, and approximately 20 recombinants within this region have been identified and are being evaluated for sensitivity to bacterial spot. If the problem is due to linkage drag, the dissociation of the two traits should significantly improve breeding efforts for bacterial spot tolerance in I-3 backgrounds.