Evaluation of growth response of phytopathogens Alternaria alternata , Diaporthe nobilis and Phytophthora plurivora to inhibitory potential of three essential oils of Monarda didyma genotypes

The growth response of three phytopathogens to inhibitory activity of essential oils (EOs), derived from three monarda ( Monarda didyma L.) genotypes, was evaluated. The antimicrobial effect was assayed by agar diffusion method. The pathogens expressed distinct growth response to the oils. As compared to colony growth of non-treated controls, all EOs remarkably suppressed Phytophthora plurivora ; EO1 inhibited Diaporthe nobilis to 31.5% and Alternaria alternata to 35.5% of the controls; EO2 restricted the growth of the latter two species to 41.3% and 50.5% from the control, respectively. The weakest inhibitory effect was established for EO3 which stopped the development of A. alternata to 67.2% and of D. nobilis to 59.1% of their controls. GC–MS analyses of the oils identified altogether 46 chemical compounds. The largest group of constituents was the group of oxygenated monoterpenes (61.6–66.0%), followed by monoterpene hydrocarbons (16.6–17.7%), non-terpenes (12.6–15.5%) and sesquiterpene hydrocarbons (2.3–3.5%). Considering the most abundant compound, EO1 and EO3 were determined of thymol and EO2 of carvacrol chemotypes. P. plurivora was equally strongly susceptible to the EOs of both chemotypes, whereas the behavior of the other two species suggested different susceptibility to the prevailing EO constituents. The total antioxidant activity was very high but differed between the oils. The obtained results add novel information on the inhibitory activity of EOs from various M. didyma genotypes and indicate specificity of the growth response of the studied microorganisms to the chemical ingredients in the applied plant products.