Methods and compositions based upon using phenolic aromatic aldehydes (ex: cinnamaldehyde, benzaldehyde) are provided, which find use as agents for treating, preventing, or curing systemic bacterial infections of living plants, in particular against Gram negative bacteria and more particularly species of Liberibacter, including Ca. Liberibacter asiaticus. The agent compositions described are used synergistically with other antimicrobial compounds, such as those that plants manufacture or release as a result of biotic or abiotic stresses, including the application of the aldehydes, proteins, whether produced by recombinant methods or not, or by essential oils such as carvacrol or allicin. Methods of applying the compositions for agriculture use are disclosed.
