Bacterial leaf spot (BLS) of lettuce (Lactuca sativa L.) is caused by the bacterium Xanthomonas hortorum pv. vitians that is hypothesized to have at least three races of the pathogen present in North America as defined by their differential resistance phenotypes in lettuce cultivars/accessions. Although resistance to X. hortorum pv. vitians race 1 has been identified in cultivated lettuce, numerous other X. hortorum pv. vitians strains cause disease on cultivars carrying this resistance locus. Thus far, resistance to these "additional" X. hortorum pv. vitians strains has not been adequately described in L. sativa or in any other wild Lactuca species sexually compatible with cultivated lettuce. We have performed an extensive screening of approximately 500 Lactuca accessions from L. sativa, L. serriola, L. saligna, L. virosa, L. aculeata, L. altaica, and L. perennis species to identify accessions resistant to these additional X. hortorum pv. vitians races. Following the initial screenings, greenhouse tests confirmed that X. hortorum pv. vitians race 2 and race 3 could be defined using L. serriola accessions. Race 2 strain BS3127 had an incompatible response (hypersensitive response) on 10 L. serriola accessions, including PI 491114 and PI 491108, whereas race 1 (BS0347) and race 3 (BS2861) strains of X. hortorum pv. vitians showed a compatible response (disease) on these genotypes. L. serriola accession ARM09-161 (and selections derived from it) was the only genotype resistant to the race 3 strain BS2861. L. serriola accessions identified in this study to be resistant to race 2 and race 3 of X. hortorum pv. vitians, together with race 1-resistant cultivars, can be used for pyramiding resistance loci against the three races of the BLS-causing pathogen.
Keywords: Lactuca sativa; bacterial diseases; breeding; disease control; disease resistance; genetic diversity; hypersensitive response; race specificity; specialty crops.