Molecular breeding for tomato bacterial spot caused by Xanthomonas spp. has been constrained by several factors. First, four species and five races of the pathogens have been described. The causal agents include X. euvesicatoria (race T1), X. vesicatoria (race T2), X. perforans (race T3, T4, T5), and X. gardneri (race T2) (Jones et al. 2004,2005). Secondly, resistance traits in several known sources are quantitatively inherited. Thirdly, the three resistance sources Hawaii 7998, Hawaii 7981, and PI 114490 are either S. lycopersicum or S. lycopersicum var. cerasiforme, which are genetically highly similar to cultivars. Mapping results for resistance are summarized in Table 12. Hypersensitive resistance to race T1 was found in Hawaii 7998, which is associated with at least three QTLs (Yu et al. 1995). However, the hypersensitive response of Hawaii 7998 only contributed partially to field resistance. Therefore, evaluation of resistance reactions was later based on symptom severity on seedlings or in the field (Yang et al. 2005b). Molecular marker Rx3-L1 has been used in MAS for selecting Rx3 and its efficacy has been demonstrated (Yang et al. 2005a). In locations where race T3 and T4 are predominant, Xv4 would be a useful resistance gene. Markers TG599 and TG134 were located 8.4 or 11.9 cM from Xv4 (Astua-Monge et al. 2000). Additional efforts in developing more closely linked markers would be necessary for applying MAS. This should be tractable because the origin of Xv4 is S. pennellii (Table 12). The Bs4 gene identified in cv. Money Maker (Ballvora et al. 2001) would not be useful in controlling bacterial spot on tomato, because it interacts with avrBs4, which is present in Xanthomonas strains that do not infect tomato (Bonas et al. 1993). Resistance to multiple races was found in PI 114490 and resistance to all races was not controlled by the same genes. However, a region on chromosome 11 was held in common for resistance to races T2, T3 and T4. The challenge to find useful resistance markers from PI 114490 has been the low polymorphism among S. lycopersicum lines.
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