A method for promoting the health of a plant comprises administering malic acid to the plant or the soil in an amount effective to recruit plant growth promoting rhizobacteria (PGPR) to the plant. Administration of malic acid promotes biofilm formation of PGPR on the plant's roots, thereby restricting entry of a foliar pathogen through stomatal pores present in the leaves. Another method for promoting the health of a plant comprises administering acetoin to the plant or the soil in an amount effective to increase pathogen resistance in aerial parts of the plant.