Studies on resistance to beet western yellows virus in lettuce (Lactuca sativa) and the occurrence of field sources of the virus

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Abstract
An effective method based on glasshouse and field procedures was developed for screening commercial cultivars and other lettuce types for resistance to beet western yellows virus (BWYV). Field experiments in 1985, 1986 and 1987 showed that lettuce cultivars varied in their reaction to BWYV, but no high levels of resistance were identified in the main commercial types. Crisp types generally showed milder symptoms than butterhead or cos types, but individual butterhead cultivars were identified with resistance equal to the best crisp types. The highest levels of resistance were identified in Batavian type cultivars and extreme resistance or possible immunity was found in Lactuca perennis and L. muralis. BWYV caused yield reductions in some cultivars as high as 63% and reduced maturation by up to 38%, in others. There was no correlation between chlorotic leaf symptom severity and yield reduction.BWYV was isolated from a range of weed and non‐lettuce host species growing near affected lettuce crops. Isolates of BWYV obtained from infected lettuce and brassicas appeared to be similar. They infected sugar beet with difficulty but caused no symptoms, and could only be detected by ELISA serology.