Evaluation of Lettuce Germplasm for Response to Bacterial Leaf Spot

A new method was developed for evaluation of lettuce germplasm for response to Xanthomonas campestris pv. vitians, the pathogen of bacterial leaf spot disease of lettuce.  This method has the characteristics of (1) allowing a large number of lettuce plants to be tested at a time in a small space, (2) maintaining high humidity that is favorable for disease establishment and development, (3) using one application of inoculum that eliminates the potential confounding effects caused by multiple inoculations used in other methods, and (4) saving time because the testing is completed in a short period of time (6–7 weeks). Sixty nine germplasm lines and ten commercial cultivars representing crisphead, romaine, and butterhead lettuce were evaluated using this method. There were no lines exhibiting resistance stronger than Little Gem, the resistant control, however, several lines did not differ significantly from Little Gem for disease severity. All commercial cultivars were susceptible or moderately susceptible to X. c. vitians. In this study, Little Gem was only moderately resistant. Our study demonstrated that high resistance to bacterial leaf spot is rare in lettuce. Screening additional germplasm for high resistance is under the way in our laboratory.