Antifungal Activity Of Citronella Essential Oil Against The Main Pathogens Of Cherry Tomato

Postharvest losses of fruit and vegetables are enormous which caused by plant relating pathogenic fungi. Currently, chemical synthetic fungicides are mostly used as an effective postharvest disease control strategy, however, there are plenty drawbacks of chemical synthetic fungicides such as long degradation time, teratogenic and carcinogenic properties and resistance to various types of fungicides. Therefore, natural, safe, effective, eco-friendly strategies need to be developed as an alternative to synthetic fungicides for controlling postharvest diseases. Essential oil, aromatic volatile oily liquid of plant secondary metabolism, has low mammalian toxicity, a broad spectrum of antifungal activities and eco-friendly property. It attracts lots of attention to study. Citronella oil, extracted from Cymbopogon nardus L. Rendle, exhibits a strong antimicrobial activity. Because of the lack of information about the antifungal activity of citronella oil, this research systematically evaluate the efficacy of antifungal activity against the main pathogen of cherry tomato—Botrytis cinerea and Alternaria alternata, and discuss the mode of action of citronella oil.In this study, three essential oils were conducted to test the antifungal activity against B. cinerea and A. alternata in vitro and in vivo to screen out oil with the strongest activity, and the results turned to be citronella oil. Thus, citronella oil was selected to study the antifungal activity systematically against B. cinerea and A. alternata in vitro and in vivo. The main results were showed as below:1. The screening test of three essential oils against B. cinerea and A. alternata in vitro and in vivo.Three essential oils were conducted against B. cinerea and A. alternata in vitro (through fumigation and direct contact method) and in vivo (cherry tomato). The results showed that citronella oil exhibited strongest antifungal activity. Through direct contact:the mycelial growth of the fungi was completely inhibited at oil concentration of1.5μl/ml. Through fumigation:the inhibition of B. cinerea and A. alternata were84.56%and80.39%respectively when the oil dosage was15μl. In vivo:the inhibition of grey mold and black rot in cherry tomato was50.33%and61.03%respectively.2. The inhibition of citronella oil against B. cinerea and A. alternata and the determination of MIC in PDA medium.Different concentrations of the oil were conducted to inhibit the mycelial growth of the fungi through direct contact and fumigation in PDA medium. The results showed: through direct contact:the mycelial growth of B. cinerea and A. alternata were completely inhibited at oil concentrations of1.5μl/mlå'Œ1.0μl/ml respectively. Through fumigation:the inhibition of B. cinerea and A. alternata were92.4%and100%respectively when the oil dosage was15μl.3. Effect of citronella oil against B. cinerea and A. alternata on spore germination, wet and dry mycelium weightDifferent concentration of citronella oil were conducted to inhibit the spore germination, wet and dry mycelium weight in PDB medium, the results showed: citronella oil inhibited fugal spore germination significantly compared to the control in a dose-dependent manner. The spore germination of B. cinerea was completely inhibited at0.8μl/ml and the inhibition of A. alternata was92.48%. In addition, the biomass of the fungal mycelium for treated by oil were significantly (p<0.05) reduced compared to the control. The biamass of B. cinerea and A. alternata mycelium was0at1.2μl/ml and0.8μl/ml respectively. Thus, the MIC of B. cinerea and A. alternata in PDB medium was1.2μl/ml and0.8μl/ml respectively.4. Antifungal activity of the essential oil in vivo on cherry tomato and the effect of fruit qualityDifferent concentrations of citronella oil were conducted to inhibit grey mold and black rot in cherry tomato. The results showed:citronella oil could effectively inhibit B. cinerea and A. alternata and the effective dosage of the oil were0.9μl/mlå'Œ1.5μl/ml respectively. The mass loss, fruit firmness, total soluble solids and titratable acidity of the fruit untreated/treated by the oil were determined and the results showed that the oil exhibited no negative impact on cherry tomato quality which provided security guarantees to practical application.5. The mode of action of citronella oil against B. cinerea and A. alternataThe microstructures of the fungi untreated/treated by the oil were observed through SEM. Total DNAs and proteins of the fungi were extracted to explore the mode of the antifungal action preliminarily. The results showed:the control group exhibited regular and normal morphology, plump hyphae with smooth external surface with rich conidia. In contrast, the hyphae of treated fungi revealed abnormal mycelial morphology with shrivelled and wrinkled flattened empty hyphae and are deformed with absence of conidia. In addition, the fungi cell extravasation rate was significantly increased after treated by the oil. Total amount of DNA had no significant difference between the control and treatments while the protein contents showed significant difference. This indicated that citronella oil damaged fungi cell membranes, made protein leakage and lead to the increasing of cell extravasation rate.6. Chemical composition of the essential oilFor detecting the active ingredients in the oil, the chemical composition of citronella oil was analysed by GC-MS. The results showed:A complex mixture of23compounds totally accounting for96.68%of the oil were identified, including monoterpenes (75.38%) and sesquiterpenes (21.30%) which were mainly citronellal (26.23%), geraniol (19.5%), citronellol (12.96%).In summary, citronella oil exhibits strong antifungal activity against B. cinerea and A. alternata and has no negative effect on fruit quality of cherry tomato. Citronella oil could be a promising natural product to be used as anti-B. cinerea and anti-A alternata agent to control the black rot in cherry tomato.