Relationship of Turnip Mosaic Virus Susceptibility and Downy Mildew (Bremia lactucae) Resistance in Lettuce1

Abstract

Abstract A mosaic disease of Lactuca sativa L. is described and the causal agent identified as turnip mosaic virus (TuMV). Extensive infection reduces the yield appreciably or may destroy entirely the value of the crop. A survey of L. sativa cultivars indicated that TuMV susceptibility is restricted to mildew resistant: crisphead types: ‘Calmar’, ‘E-4’, ‘Imperial 410’, ‘Imperial Triumph’, ‘Valrio’, ‘Valtemp’, and ‘Valverde’. Circumstantial evidence indicates that TuMV susceptibility in cv. ‘Calmar’, ‘Imperial 410’, ‘Valrio’, ‘Valtemp’, and ‘Valverde’ stems from the mildew resistant L. serriola L. (P.I. 91532). TuMV and mildew resistant cultivars are: butterhead type ‘May King’, ‘Meikoningin’, ‘Proeftuin’s Blackpool’, ‘Ventura’; leaf type ‘Red Salad Bowl’, ‘Salad Trim’; cos type ‘Valmaine’. Seed collections of L. serriola from the Santa Clara and Salinas Valleys of California produced plants that fell into 3 classifications: a) TuMV-resistant, mildew-resistant; b) TuMV-resistant, mildew-susceptible, and c) TuMV-susceptible, mildew-resistant. No plants in L. sativa or L. serriola were susceptible to both TuMV and mildew. Extreme resistance to TuMV was demontrated in L. sativa and L. serriola . TuMV-susceptible L. sativa cultivars showed differences in tolerance to symptom expression and resistance to infection. In L. serriola a resistance connected with a hypersensitivity reaction was observed. All isolates of TuMV collected were capable of infecting susceptible L. sativa cultivars. L. sativa cv. ‘Calmar’ and ‘Valverde’ systemically infected with TuMV did not transmit the virus through the seed. L. serriola systemically infected with TuMV failed to produce seed.