Technical Abstract: Outbreaks of a bacterial blight disease occurred on garlic (Allium sativum) cultivars Roxo Caxiense, Quiteria and Cacador in Southern Brazil, and threatened the main production regions of Rio Grande do Sul State. Symptoms were characterized by watersoaked reddish streaks along the leaf midrib, followed by yellowing of leaves, a rot of bulbs and plant death. The disease can negatively impact seed production in infested fields. Epidemics occurred mainly during bulb formation and the preharvest period. Bacteria, fluorescent on King’s B medium, were isolated from leaf tissue. Physiological tests indicated that the bacteria were Pseudomonas marginalis. Pathogenicity tests were performed on plantlets and detached cloves. Symptoms similar to those in the field were reproduced on the leaves. Additionally the bacteria incited a rot of cloves. Reisolates had physiological properties identical to the inoculated strains, demonstrating Koch’s postulates. DNA fragment pattern analysis using DNA amplified with the BOXA1R primer, demonstrated that two different pathogens were responsible for the disease. The DNA fragment patterns were different than the type strain of P. marginalis and the pathotype strains P. marginalis pv. marginalis, P. marginalis pv. pastinaceae, P. marginalis pv. alfafae. Although the strains appear to be similar to P. marginalis according to physiological tests, further research is needed to determine if the bacteria represent a novel pathovar or species.