Most of the lettuce grown in the USA comes from Arizona and California. In recent years, both locations have experienced either excess rain, heat or unseasonal shifts which delayed plantings or caused early harvests. A 2018 outbreak of bacterial contamination on Romaine lettuce is a particular red flag. Extreme weather appears to have weakened the resistance of lettuce to E coli. Farming has always been subject to unpredictable conditions, but extreme weather events are becoming more common. Plant breeders are attempting to use genetic editing to create more resistant varieties.