Lettuce dieback disease is widespread in commercially grown romaine and leaf-type lettuces, but not in iceberg-type cultivars. The disease is caused by two closely related Tombusviruses: Tomato bushy stunt virus (TBSV) and Lettuce necrotic stunt virus (LNSV). A single dominant gene (Tvr1) on chromosomal linkage group 2 has been found to be responsible for the resistance observed in modern iceberg lettuces. A population of 192 F8 recombinant inbred lines (RILs) developed from crosses between the susceptible romaine cultivar 'Valmaine' and the resistant iceberg cultivars 'Salinas 88' and 'Salinas' was used to pinpoint the position of the resistance gene with EST-based molecular markers. Nine markers closely linked to the Tvr1 gene were subsequently tested for association with resistance in a set of 68 accessions from all horticultural types of lettuce. Sequencing of the marker that matched precisely with resistance revealed the presence of three haplotypes in L. sativa, two of them associated with resistance. High-resolution DNA melting analysis, which allows detection of all three haplotypes in a single analysis, was successfully applied to study the marker-trait association. Results confirmed the usefulness of the marker for marker-assisted selection in all types of cultivated lettuce.