In 5 field experiments on land infested with lettuce big-vein disease (LBVD) at different locations, the incidence of LBVD in partially resistant lettuce breeding line LE169 was always diminished significantly compared with its incidences in susceptible crisphead lettuce genotypes. Also, the fresh weight yield of LE169 was always significantly greater than those of more susceptible genotypes. When different genotypes were grown on virgin land where infested introduced lettuce seedlings were the only source of the disease, LBVD incidence was again significantly diminished and yield significantly greater with LE169. When black plastic mulch was spread on the surface of LBVD-infested soil and LE169 was deployed along with more susceptible lettuce genotypes, presence of mulch and partial resistance both diminished LBVD incidence significantly on their own, but combining the two suppressed spread the most. The mulch diminished soil moisture and increased soil temperature, one or both of which decreased activity of the viruliferous zoospores of its Olpidium brassicae vector, resulting in less virus transmission to lettuce roots.