Abstract: Improved methods and compositions for protecting plants or seeds from plant diseases are provided. The compositions comprise (and preferably consist essentially of) saponin, such as triterpene type saponins extracted from quinoa or quillaja. Compositions according to the invention may comprise (or consist essentially of) a quantity of a sapogenin compound, preferably a sapogenins selected from the group consisting of oleanolic acid, hederagenin, phytolaccinic acid, and quillaic acid. The methods comprise contacting the portion of the plant (e.g., foliage, shoot, etc.) to be treated with a disease-inhibiting or protective amount of the composition. The compositions can also be used to treat plant seeds or tubers prior to planting thereof, as well as soil adjacent a growing plant. The inventive compositions are particularly useful for the treatment, control and/or prevention of bacterial diseases.