Technical Abstract: Xanthomonas leaf blight (Xanthomonas axonopodis pv. allii) is a yield-limiting disease of onion in the western U.S. Frequent applications of copper-based bactericides amended with an ethylenebisdithiocarbamate fungicide (e.g., maneb or mancozeb, class B2 carcinogens) provide some disease suppression, but strategies to reduce conventional bactericide use are needed to minimize grower costs, environmental impacts, and public exposure to class B2 pesticides. Applications of acibenzolar-S-methyl (Actigard 50WG) reduced in planta and epiphytic populations of X. axonopodis pv. allii as effectively as applications of copper hydroxide-mancozeb (ManKocide) in growth chamber studies. Under field conditions, four weekly applications of acibenzolar-S-methyl reduced severity of Xanthomonas leaf blight as effectively or more than nine to 12 weekly applications of copper hydroxide (Kocide) or copper hydroxide-mancozeb. Acibenzolar-S-methyl applications did not increase bulb yield or grade compared to copper bactericide treatments. However, bulb yield was reduced 22 to 27% when 10 weekly applications of acibenzolar-S-methyl were made in the absence of disease. Application of a commercial formulation of both Pantoea agglomerans strain C9-1 and Pseudomonas fluorescens strain A506 (BlightBan A506/C9-1) reduced severity of Xanthomonas leaf blight in field experiments. Weekly copper hydroxide applications starting one to two weeks before bulb initiation were as effective as weekly applications started three to four weeks before bulb initiation, irrespective of the maneb rate used. Integration of acibenzolar-S-methyl and biological control agents with copper hydroxide in a carefully-timed spray program may eliminate the use of the class B2 carcinogens maneb and mancozeb on onion without compromising efficacy for management of Xanthomonas leaf blight.