Differential Sources of Resistance from Against Three Races of pv. Causing Bacterial Leaf Spot of Lettuce.

Bacterial leaf spot (BLS) of lettuce ( L.) is caused by the bacterium pv. 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 pv race 1 has been identified in cultivated lettuce, numerous other pv strains cause disease on cultivars carrying this resistance locus. Thus far, resistance to these "additional" pv. strains has not been adequately described in or in any other wild species sexually compatible with cultivated lettuce. We have performed an extensive screening of approximately 500 accessions from , , , , , , and species to identify accessions resistant to these additional pv races. Following the initial screenings, greenhouse tests confirmed that pv race 2 and race 3 could be defined using accessions. Race 2 strain BS3127 had an incompatible response (hypersensitive response) on 10 accessions, including PI 491114 and PI 491108, whereas race 1 (BS0347) and race 3 (BS2861) strains of pv showed a compatible response (disease) on these genotypes. accession ARM09-161 (and selections derived from it) was the only genotype resistant to the race 3 strain BS2861. accessions identified in this study to be resistant to race 2 and race 3 of pv, together with race 1-resistant cultivars, can be used for pyramiding resistance loci against the three races of the BLS-causing pathogen.