SUMMARYThe synthetic auxinlike postemergent herbicides 2,4-D, 2,4,5-T, 2,4,5-TP, dicamba, and MCPP were examined for their influence on conidial germination, germ-tube growth, primary branching of germ tubes, and conidial production ofDrechslera sorokinianaat molar concentrations of 10-12to 10-3. All herbicides of the 10-3M concentration prohibited conidial germination and caused visible plasmolysis of conidium cells. Germination of conidia was not affected by any herbicides at concentrations of 10-12to 10-4M. Germ-tube growth and primary branching of germ tubes were stimulated by most herbicides at concentrations of 10-12to 10-4M; some nonsignificant inhibition of these structures occurred in response to 2,4,5-TP and MCPP. Mycelial growth was stimulated by most herbicides at concentrations of 10-12to 10-4M; significant increases within this concentration range, however, differed with specific herbicides. Conidial production was inhibited by 2,4,5-T and MCPP at all concentrations; 2,4,5-TP and dicamba caused significant increases in conidial production at lower concentrations and a significant decrease at higher concentrations. Only 2,4-D increased conidial production at higher concentrations. The results of the investigation indicate that germ-tube growth is most dramatically stimulated by the herbicides examined and that vegetative growth (germ tubes, branching, mycelium) ofD. sorokinianashows a greater stimulatory response to very low concentrations of the herbicides. The study shows also that the herbicides at their various concentrations may have opposite effects on growth ofD. sorokiniana, depending on the stage of growth of the organism.