In Quebec, bacterial leaf spot of lettuce was observed for the first time in 1994. Since this first mention, the disease has been observed each year and the severity varied with environmental conditions. Little information was available on this disease because until recently it was only sporadically observed around the world following the first mention in 1918. In this project, we found that two groups of Xanthomonas caused the bacterial leaf spot of lettuce according to the BIOLOG profiles. From the results of strain characterization, a semi-selective medium has been developed to detect and quantify X. campestris pv. vitians. This medium is made of maltose, tryptone, methyl green, phosphate salts, amoxicillin, cephalothin, cycloheximide and trace elements. It allowed us to carry out studies on the ecology of the pathogen and on the disease epidemiology. The effect of weather conditions on bacterial population size and the bacterial leaf spot development has been studied. Weather parameters influencing the bacterial population were the number of hours with temperature higher than 28°C, the number of hours with wind velocity lower than 1 km per hour, the number of hours with relative humidity lower than 45% and the minimum relative humidity. The weather parameters that significantly discriminated between disease increase categories were the mean solar radiation, the number of hours with relative humidity higher than 90%, the mean relative humidity and the maximum temperature. Looking at the relationship between X. campestris pv. vitians population size and host plant development, it was shown that both bacterial population size and disease severity increased with leaf age. Mathematical models were developed to show these relationships. This information will be useful in disease management to decide when to apply bactericides and when to harvest. Finally, an exploratory study was conducted looking at the effects of nutrients on the size of saprophytic bacterial

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