Identification Of Tomato Bacterial Spot Race T1, T2, T3, T4, And Xanthomonas Gardneri Resistance Qtls Derived From Pi 114490 Populations Selected For Race T4

Conventional breeding for resistance to bacterial spot (Xanthomonas sp.) in tomato (Solanum lycopersicum L.) has been difficult due to complex genetic control and the emergence of virulent races of the pathogen. There may also be unfavorable linkages of resistance with small fruit size, late maturity, and low yield. In Florida, the predominant race is T4 but some T3 is sometimes found. We have been interested in resistance QTL from PI 114490 because it has resistance to all four field races of the pathogen. However, we have not been able to recover the level of resistance of PI 114490 in breeding lines developed from it. To improve our understanding of the genetic basis of resistance, PI 114490 was crossed to two susceptible breeding lines, Fla. 7324 and Fla. 7613, and F-2 populations were developed. In spring 2010, approximately 2600 F-2 seedlings were screened with a molecular marker at the Sp locus, and determinate plants were planted to the field and subsequently evaluated for bacterial spot race T-4 disease severity. Phenotypic evaluations and selections were carried out on 500 F-2 plants in spring 2010, on 260 F-3 selected lines in spring 2011 and on 203 F-4 selected lines in fall 2012. Directional selection for the highest or lowest levels of resistance was made at each generation. Ninety-five resistant or susceptible F-4 selections were genotyped in fall 2012 using the SolCAP array, consisting of 7,720 SNPs. There were 2,448 SNPs that were polymorphic between PI 114490 and both susceptible parents. In spring 2012, F-5 progeny lines of 92 selections were evaluated for race T4 disease severity in Florida, and these 92 lines were separately tested in Ohio for resistance to races T1, T2, T3, and X. gardneri in summer 2012. Marker-trait analysis identified seven regions with strong effects; a large region on chromosome 3 had the largest effect for all races, two regions on chromosome 2 affected all races, two regions on chromosome 10 affected all races, and two regions on chromosome 11 did not affect T3 but affected the other races, especially T4. In addition, five weaker QTL were found for all or some races, and five QTL were found where the PI 114490 allele conditioned greater susceptibility.