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Poster: Diseases of Plants: Disease Detection & Diagnosis


Three races of Xanthomonas campestris pv. vitians causing bacterial leaf spot on lettuce identified.
C. BULL (1), M. Trent (2), R. Hayes (3) (1) Department of Plant Pathology and Environmental Microbiology, Penn State University, U.S.A.; (2) USDA/ARS, U.S.A.; (3) USDA/ARS, U.S.A.

Xanthomonas campestris pv. vitians (Xcv), the causal agent of bacterial leaf spot on lettuce, reduces lettuce quality and yield worldwide. Although strain specific, host resistance is the most feasible method to reduce losses. Therefore, we screened lettuce germplasm for resistance using the broadest diversity within the pathogen population. A collection of 120 strains were assigned to one of six sequence types (A, B1, B2, C, D and E) by multilocus sequence typing (MLST) using concatenated sequences of rpoD, dnaK, fyuA, gyrB, and gap1. The abaxial sides of lettuce leaves were infiltrated with single bacterial isolates (1 x 108 CFU/ml) representing each sequence type. Tissue collapse beginning at 30 hr and resulting in light-brown papery lesions at 48 hr was considered an incompatible reaction. Replicated assays with strains were conducted with germplasm for which resistance was identified. This method was used to determine that the single dominant gene, named Xanthomonas resistance 1 (Xar1), confers complete resistance to MLSTs B, D, and E. Subsequently, germplasm was identified with complete resistance to strains from sequence type A and a separate source of resistance was identified to strains from sequence type C. The genetics of resistance to sequence types A and C is being elucidated and we are completing analysis of all strains on resistant germplasm. However, the data gathered to-date indicate that there are at least three races of Xcv.