Technical Abstract: Xanthomonas leaf blight, caused by the bacterium Xanthomonas axonopodis pv. allii, is a common foliar disease of onion. Environmental, host, and cultural factors affect the severity of the disease, but reductions in yield of 20% or greater are common. This article presents a review of disease symptomology, etiology, epidemiology, and management. The disease can be managed effectively by the integration of moderately resistant cultivars, strict sanitation of weeds, crop debris, and volunteer onion, sound cultural practices, and judicious use of chemical controls.