The effect of essential oils derived from various plant species versus the Mancozeb + Metalaxyl fungicide in inhibiting early blight of tomato

Tomato (Solanum lycopersicum L.) belongs to the Solanaceae family and is economically one of the most important vegetable crops in Iran after the potato, cultivated all year but threatened by a large number of plant pathogens. One of the most significant diseases affecting this plant is tomato early blight, caused by various species of Alternaria. To date, chemical fungicides have been the only effective method of controlling this disease. Given the damaging effects of chemical pesticides on human and environmental health, as well as food safety, this study aimed to compare the inhibitory effects of savory, garlic, thyme, fennel, and rosemary essential oils on tomato early blight disease to that of a fungicide, Mancozeb+Metalaxy, under laboratory and greenhouse conditions.

Plant essential oils were extracted using a Clevenger apparatus and, after dehydration, utilized at 50, 150, and 250 μL/L concentrations by mixing with cultural media in Petri dishes. The growth inhibitory effect of plant essential oils was assessed by measuring the fungal colony’s growth diameter and calculating the amount of Minimum Inhibitory Concentration (MIC) and Minimum Fungicidal Concentration (MFC) associated with each essential oil. Moreover, the effect of plant essential oils on tomato early blight control was evaluated at a 750 μL/L concentration by spraying tomato plants with 3 to 5 leaves under greenhouse conditions. A chemical mixture containing Mancozeb and Metalaxyl was used as a positive control to compare its antifungal effects versus plant essential oils.

Statistical analysis using Duncan’s multiple-range test revealed a significant difference in all treatments at the 5 percent level. A. alternata growth diameter measurements showed that the lowest colony growth diameter was associated with 250 μL/L of savory essential oil with 100% inhibition. At a concentration of 50 μL/L, rosemary essential oil inhibited fungus growth by 6.35%, resulting in the highest average growth diameter. Accordingly, among the studied essential oils, savory exhibited the strongest inhibitory effect, while rosemary yielded the weakest. However, MFC determination revealed that garlic essential oil demonstrated the highest fungicidal properties and the lowest amount of MFC (700 μL/L). On the other hand, fennel essential oil yielded the highest amount of MFC (2000 μL/L). In general, all essential oils were able to deteriorate and disrupt the structure and morphology of A. alternata mycelium, leading to sedimentation and granulation of cytoplasmic contents and its organelles. This fungus showed the least and most sporulation in the savory and rosemary treatments, respectively, compared to the control. In greenhouse studies, unlike laboratory tests, the highest control rate of tomato early blight was related to the application of a fungicide with an average leaf surface contamination of 1.6%, which demonstrated the highest control rate in greenhouse conditions compared to other essential oil treatments. Compared to the infected control, savory essential oil and rosemary essential oil, with disease severity of 16.66% and 35.0%, respectively, had the greatest effect on the appearance of symptoms following fungicide application.

Numerous plant compounds have been studied for their antifungal and antibacterial properties thus far. Using the antifungal effect of plant essential oils is undoubtedly one of the newest techniques for controlling plant pathogens. The current study compares plant essential oils to a chemical fungicide for controlling tomato early blight in vitro and under greenhouse conditions and demonstrates that plant essential oils are more effective than chemical fungicides. Consistent with this finding, previous research has demonstrated the potential of plant essential oils to control early blight in tomatoes.