Chlorophyll fluorescence imaging to facilitate breeding of Bremia lactucae -resistant lettuce cultivars

•CFI clearly detected effects of downy mildew infection on photosynthesis of butterhead and batavia lettuce cultivars.•In leaf discs of susceptible cultivars, Fv/Fm decreased during days 10–14 after inoculation (dai).•On dai 14, relative cumulative Fv/Fm obtained best separation of susceptible and resistant cultivars for automatic detection.•CFI objectively distinguished susceptible and resistant butterhead and batavia lettuce cultivars.