Disclosed are novel methodologies for virulence enhancement of fungal and bacterial pathogens for biological control of target plants. Described is a selection process for phytopathogenic microorganisms that excrete selected amino acids. Pathogenicity studies demonstrate that these amino acid-excreting plant pathogens show greater virulence against target plants than do corresponding wild type strains. Host range evaluations of these mutants did not reveal any increase of virulence towards non-target plants. This novel approach to enhancement of microbial herbicides can be used across a broad spectrum of microbial groups to improve the efficacy of bio-control. Also disclosed is the use of selected mutants of plant pathogenic microorganisms that overproduce one or more inhibitory amino acids to enhance control of target plants.