The present invention provides a method for controlling crown gall disease in plants using an effective quantity of .alpha.-proteobacteria that produces trifolitoxin (TFX). The present invention also provides a biocontrol agent for use in the above method, and a plant coated with the biological control agent. The biocontrol agent is characterized as a biologically pure culture of an .alpha.-proteobacteria strain that produces TFX, or an .alpha.-proteobacteria strain genetically engineered to produce TFX. The .alpha.-proteobacteria strain employed may include any one of the many strains of Agrobacterium capable of producing crown galls, including Agrobacterium vitis and, in particular, A. vitis F2/5. The .alpha.-proteobacteria strain employed may be genetically engineered to produce TFX by introducing a genetic construct into the Agrobacterium so as to cause the Agrobacterium to carry and express the tfx operon from Rhizobium. The bacteria may also be genetically engineered to produce TFX by introducing a pT2TFXK plasmid into the Agrobacterium. The biocontrol agent may also be the strain Agrobacterium vitis F2/5 (pT2TFXK), ATCC Patent Deposit Designation PTA-2356.