Molecular Breeding for Resistance to Xanthomonas Species Causing Bacterial Spot of Tomato

Bacterial spot of tomato is caused by four species of Xanthomonas with multiple physiological races. This disease has led to severe economic losses in humid growing environments. A complex breeding population was developed using six advanced breeding lines with resistance from at least three distinct sources. Hybrids were intercrossed to create a segregating population consisting of over 1,100 progeny. In a field trial inoculated with the T1 strain, single plant selections were made for resistant and susceptible extremes of the population. Subsequent plot-based evaluations of these selections were conducted with both T1 and T3 inoculations in separate replicated field trials. Three selected lines (OH7663, OH7667 and 2k7-6117-S2) showed high levels of resistance to both strains. Using graphical genotypes developed from the SolCAP SNP array, we demonstrated that these breeding lines share a common coupling phase recombination event which brings a QTL derived from Hawaii 7998 together with Rx-4 derived from PI 128216 on chromosome 11. These lines also contain Rx-3 on chromosome 5. With additional field evaluations, we found that the three lines showed partial resistance to multiple species of bacterial spot including X. gardneri. We used a similar directional selection strategy and a set of 384 SNPs optimized for use in processing tomato germplasm to identify further loci associated with resistance to X. gardneri. These results demonstrate the efficacy of the SolCAP SNPs for breeding and genetic analysis in elite tomato populations.