Technical Abstract: The disease, lettuce dieback, causes severe losses for lettuce production in the western US and is caused by a group of tombusviruses, including both Tomato bushy stunt virus and the newly described lettuce necrotic stunt virus (LNSV). Symptoms include yellowing, necrosis, stunting and death of plants, with losses ranging from a few plants to entire crops; however, incidence in a field can vary annually. The genome of LNSV was sequenced and has an organization typical of the genus, Tombusvirus. Much of the genome is most closely related to TBSV; however, the coat protein is closely related to that of Moroccan pepper virus, a partially characterized tombusvirus from Mediterranean regions. In order to identify factors contributing to variability in infection, soil analyses were conducted on adjacent fields with similar soil type, but differing for disease incidence. Experiments were conducted to elucidate environmental factors contributing to disease development, and identified moisture and salinity as significant factors contributing to infection. Similarly, efforts to induce disease under controlled conditions demonstrated that exposure to long days and high temperatures can also induce systemic infection of lettuce, but severity varied among specific treatments. These observations demonstrate that both exposure to the pathogen and specific environmental factors are required for development of dieback symptoms on lettuce.