A Novel Screening Method for Evaluation of Lettuce Germplasm for Bacterial Leaf Spot Resistance

Bacterial leaf spot of lettuce, caused by Xanthomonas campestris pv. vitians, is a devastating disease of lettuce worldwide. Because there are no chemicals available for effective control of the disease, host-plant resistance is highly desirable to protect lettuce production. A new method for fast screening and accurate identification of bacterial leaf spot (BLS)-resistant lettuce has been developed in our laboratory. A total of 79 lettuce genotypes (69 germplasm lines and 10 adapted cultivars) were evaluated with this technique for response to X c. vitians. Disease incidences ranged from 92% to 100% and disease severities were between 1.6 and 3.6 on the 0 to 4 scale. No highly resistant genotypes were identified. However, 12 genotypes did not significantly differ for disease severities from the moderately resistant 'Little Gem' lettuce that was used as a resistant control. Comparison of disease severities of 10 commercial cultivars and three moderately resistant germplasm lines tested at the seedling stage and adult stage showed a high positive correlation (r = 0.87, P<0.0001) between tests. The new screening method should be useful in breeding programs, in which great numbers of plants need to be tested during germplasm evaluation, and for single plant selection as well as other studies. The identification of new sources of moderate resistance in this study could facilitate development of cultivars with a higher level of resistance through the gene pyramiding approach.