Relationship of a Non-Lethal Reaction to a Virulent Isolate of Lettuce Mosaic Virus and Turnip Mosaic Susceptibility in Lettuce1

A mosaic disease of Lactuca sativa L. is described and the causal agent identified as a new virulent isolate of lettuce mosaic virus (LMV). The reservoir of infection was bristly oxtongue, Picris echioides L. Lactuca sativa L. cvs. Gallega, Calmar, Imperial 410, and Bibb systemically infected with virulent LMV did not transmit the virus through the seed. A survey of L. sativa cultivars indicated that the non-lethal reaction to the virulent isolate was restricted in the crisphead type to cultivars that are turnip mosaic virus (TuMV) susceptible, and downy mildew-resistant. A similar relationship was found in L. serriola lines. The non-lethal reaction was conferred by dominant complementary genes. ‘Gallega’, reported to be resistant to common LMV, was found to be susceptible to systemic infection by the virulent LMV isolate.