BLSL has been commonly observed in Quebec since 1994. The disease seems to be favored by hot and rainy weather. Populations of Xcv and disease development were monitored in relation to weather on 5 lettuce crops over 3 summers. Inoculated plants were evaluated for Xcv population sizes (BP) (CFU/g of leaf) and disease severity (DS) (scale 0-6). Means and sums of weather data were calculated for each interval, resulting in 20 weather variables (WV). BP and DS were grouped by cluster analysis into 3 and 4 categories respectively. Stepwise discriminant analysis was conducted to statistically identify the WV that influenced the BP and the DS. WV that discriminated among categories of BP were number of hours >28 C, number of hours with wind velocity <1 km/h, number of hours with RH <45% and minimum RH. The WV that significantly discriminated between DS categories were mean solar radiation, number of hours with RH >90%, mean RH and maximum temperature.