Adebayo, O., Bélanger, A. and Khanizadeh, S. 2013. Variable inhibitory activities of essential oils of threeMonardaspecies on the growth ofBotrytis cinerea. Can. J. Plant Sci. 93: 987-995. The antifungal effects of the essential oils of three Monarda species (Monarda didyma L., M. fistulosa L., and M. didyma var. 80-1A L.) were evaluated against Botrytis cinerea Pers.: Fr. The inhibitory effects of the volatile and contact phases of the essential oils were determined on hyphae and spores. The effects of the essential oils on the morphological structures of hyphae were studied under light microscopy. All the three tested essential oils types had inhibitory effects on both mycelia and spores as follows, in descending order of effectiveness: M. didyma > M. didyma var. 80-1A > M. fistulosa. For all essential oils, the volatile phase effects were more toxic on fungal growth than the contact phase effects. All the essential oils in the contact studies achieved complete inhibition of mycelial growth and spore germination at 819.2 µg mL^-1 of ethanol and Tween 20 mixture. In the volatile studies, however, complete mycelial inhibition was achieved with M. didyma and M. didyma var. 80-1A at 0.1 µg mL^-1 air and with M. fistulosa at 0.2 µg mL^-1 air. Spore germination was inhibited at 0.05 µg mL^-1 air with M. didyma and 0.2 µg mL^-1 air with both M. didyma var. 80-1A and M. fistulosa. This study demonstrated that these essential oils are promising potential antifungal agents that could be used in the development of biofungicide alternatives to synthetic fungicides against B. cinerea.