Technical Abstract: Lettuce dieback, a disease of lettuce primarily associated with romaine and leaf lettuce types, has been identified and confirmed in most California and Arizona production areas. This new disease, known as lettuce dieback, is particularly important to the lettuce industry because of the significant increase in leaf and romaine lettuce acreage over the past several years. Two tombusviruses, Tomato bushy stunt virus (TBSV) and the closely related and newly described Lettuce necrotic stunt virus (LNSV), have been proven to cause this necrosis-inducing disease. The movement and stability of the virus in irrigation water, along with its soil-borne nature, poses a real threat of increased incidence of this disease through movement to additional areas and long-term persistence in the soil. Research has identified elevated soil salinity as a possible factor contributing to increased disease incidence in fields of susceptible lettuce. Susceptibility to lettuce dieback is widespread in the germplasm of cultivated lettuce. A single dominant gene, Tvrl, was found to be responsible for resistance in both modern iceberg lettuces and in recently released resistant romaine germplasm, and these are likely to be the same gene. We identified the location within the lettuce genome for Tvrl, the resistance gene in iceberg lettuces, and found molecular markers near the gene, which will assist breeders in moving resistance into other varieties of lettuce.