Identification of Molecular Markers Linked to Bacterial Spot Race T4 Resistance from the Solanum lycopersicum Var. Cerasiforme Accession PI 114490 QTL: Toward Developing Broad-spectrum Resistance

The development of resistance to bacterial spot (Xanthomonas spp.) in tomato (Solanum lycopersicum) has been particularly challenging, particularly because of the emergence of new races of the pathogen that overcome specific resistance mechanisms. In response, breeding efforts at the University of Florida have shifted to focus more on quantitative, race non-specific resistance. The S. lycopersicum var. cerasiforme accession PI 114490 has a high level of resistance against all known races of bacterial spot and may be a source of novel resistance loci. PI 114490 was crossed to two susceptible breeding lines, Fla. 7324 and Fla. 7613, and F₂ populations were developed. In Spring 2010, approximately 2600 F₂ seedlings were screened with a molecular marker at the Sp locus, and 500 determinate plants were planted to the field and subsequently evaluated for bacterial spot disease severity. Phenotypic evaluations and selections were carried out on 500 F₂ progeny in Spring 2010, on 260 F₃ progeny lines in Spring 2011, and on 200 F₄ progeny lines in Fall 2012, always selecting for highest or lowest levels of resistance. Ninety highly resistant or highly susceptible F_2:4 plants were selectively genotyped in Fall 2012 using the “SolCAP” SNP array, consisting of 7,720 SNPs. 2449 of these SNPs were polymorphic between PI 114490 and both susceptible parents. Marker-trait analysis identified seven regions associated with resistance, including two previously identified QTL.