Technical Abstract: To quantify the weather factors that influence BLSL development, leaf-surface populations of Xanthomonas campestris pv. vitians and development of BLSL were monitored in relation to weather parameters. Trails were conducted in July of 1998 and 1999; and during June and August of 2000 and 2001. Lettuce seedlings were inoculated with the pathogenic bacteria 24 hours befroe transplanting. Three times a week, bacterial populations were assessed on four lettuce samples and disease severity was evaluated on the same nine lettuce heads throughout the experiment. Means, sums and duration of the original weather data were calculated for each interval between sampling dates (20 new variables). The dependant variables, deviation of the expected bacterial population size (BP) and disease progress (DP), were grouped based on cluster analysis into three and four categories respectively. This grouping was done in order to identify weather patterns that influenced BP and DP. Stepwise discriminant analysis was conducted on pooled data to statistically identify the weather variables that influenced BP and DP. Based on this analysis, weather variables that discriminated between categories of BP were the number of hours with temperature >8 degrees C, number of hours with wind velocity <1 km hminus 1, number of hours with relative humidity <45% and minumum relative humidity. Variables that significantly discriminated between DP categories were mean solar radiation, number of hours with relative humidy >90%, mean relative humidity and maximum air temperature.