Because the substances to be tested are in the air, I advise to use microclimate saturation principle.
Prepare you culture medium, inoculate with your bacteria. Put the fumes source (the crushed plant material with crude extract) in small and closed environment with the inoculated medium (petri dish open). You should take in consideration the sterile manipulation.
You can evaluate the effect of fumes compared to control (without fumes) using colonies counting or other evaluation of growth available for you.