Main components of three plant essential oils and their antimicrobial activity on Phytophthora nicotianae

【Objective】 This study aimed to investigate the main components of essential oils of Eugenia caryophyllata， Litsea cubeba， and Eucalyptus citriodora， and their antimicrobial activities against Phytophthora nicotianae， so as to provide a theoretical basis for developing plant-derived pesticides for controlling tobacco black shank.【Method】 Gas chromatography-mass spectrometry （GC-MS） was used to analyze the main components of the essential oils of Eugenia caryophyllata， Litsea cubeba， and Eucalyptus citriodora. Mycelial growth rate assay was applied to investigate antimicrobial activities of the three plant essential oils and their main components against Phytophthora nicotianae. Scanning electron microscope （SEM） and transmission electron microscope （TEM） were used to observe effects of the three plant essential oils on the hyphal morphology and subcellular structures of Phytophthora nicotianae. Control effects of the main components of the three plant essential oils against tobacco black shank was investigated in a pot experiment.【Result】 The main component of Eugenia caryophyllata essential oil was eugenol， with its relative content of 82.93%； the main component of Litsea cubeba essential oil was citral， and the relative content of cis-citral and trans-citral were 28.48% and 42.41%； the main component of Eucalyptus citriodora essential oil was citronellal， with its relative content of 79.27%. The three plant essential oils and their main components showed obvious inhibitory effects on Phytophthora nicotianae： the median effective concentration （EC₅₀） values of essential oils of Eugenia caryophyllata， Litsea cubeba， and Eucalyptus citriodora on Phytophthora nicotianae were 191.5， 197.3， and 185.9 μg/mL， respectively； the EC₅₀ values of eugenol， citral， and eugenol on Phytophthora nicotianae were 111.1， 128.4， and 141.1 μg/mL， respectively. After treatments of three plant essential oils， SEM observation found that Phytophthora nicotianae hyphae exhibited fragmentation and deformation， with increased branching forming conidia， and the hyphal knots became swollen， with vacuoles appearing internally. TEM observation found that the cell membrane and mitochondria membrane of Phytophthora nicotianae degraded， with disappeared mitochondrial cristae and generated autophagic vesicles， as the cells underwent apoptosis. The pot experiment results showed that on 7 d of Phytophthora nicotianae inoculation eugenol， citral， and citronellal exhi-bited control effects on tobacco black shank of 67.8%， 50.8%， and 48.8%， respectively.【Conclusion】 Essential oils of Eugenia caryophyllata， Litsea cubeba， and Eucalyptus citriodora show favourable antimicrobial activities against Phytophthora nicotianae， and their antimicrobial mechanism may relate to the ability of these three plant essential oils to disrupt the cellular structure of Phytophthora nicotianae. The principal components of these three plant essential oils—eugenol， citral， and citronellal—demonstrate effective control against tobacco black shank， suggesting their potential to be deve-loped as plant-derived pesticides.