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Host Resistance to Mirafiori lettuce big-vein virus and Lettuce big-vein associated virus and Virus Sequence Diversity and Frequency in California

Big vein is an economically damaging disease of lettuce (Lactuca sativa) caused by the Olpidium brassicae-vectored Mirafiori lettuce big-vein virus (MLBVV). Lettuce big-vein associated virus (LBVaV) is also frequently identified in symptomatic plants, but no causal relationship has been demonstrated. Although big vein is a perennial problem in the United States, the extent of MLBVV and LBVaV infection and diversity is unknown. Lettuce cultivars partially resistant to big vein reduce losses, but do not eliminate disease. While Lactuca virosa does not develop big vein symptoms, it has not been tested for infection with MLBVV or LBVaV. Lettuce cultivars Great Lakes 65, Pavane, Margarita, and L. virosa accession IVT280 were evaluated for big vein incidence and virus infection in inoculated greenhouse trials. Additional lettuce samples were collected from field sites in California, classified for symptom severity, and evaluated for virus infection. Reverse transcription-polymerase chain reaction and nucleotide sequencing were used to determine infection with MLBVV and LBVaV, and sequence diversity among viral isolates, respectively. Infections with MLBVV and MLBVV/LBVaV were dependent on big vein symptom expression in California production areas, and isolates were closely related to those found in Europe and Japan. Partial big vein resistance was identified in Margarita and Pavane; however, MLBVV infection was found in asymptomatic plants. L. virosa IVT280 remained symptomless and virus free, suggesting that it is immune to MLBVV and LBVaV.