Provided are methods of using compositions comprising microparticulate suspensions of bismuth-thiol (BT) compounds, wherein the volumetric mean diameter of the microparticles is from 0.4-10µm, as antiseptic formulations for plants. Further provided are methods also including the use of antibiotics that are synergistic or enhance the effect of the compositions comprising microparticulate suspensions of bismuth-thiol (BT) compounds.