Antibacterial Activity and Mechanism of Litsea cubeba Essential Oil against Penicillium italicum in Citrus Fruits

Litsea cubeba essential oil (LCEO) has attracted wide attention as a natural antibacterial material due to its advantages of high safety, no drug resistance and broad-spectrum antibacterial activity. The purpose of this study was to investigate the antibacterial activity of LCEO against Penicillium italicum in citrus fruits and its mechanism. The minimum inhibitory concentration (MIC) of LCEO, and its influence on the mycelial growth, spore germination, release of cell components, extracellular pH and electrical conductivity of Penicillium italicum were determined after LCEO treatment. The changes of the surface morphology of Penicillium italiana mycelia caused by LCEO were observed by scanning electron microscopy (SEM), and the integrity of the spore membrane was detected by propidium iodide (PI) staining. The disease incidence and lesion diameter in citrus fruits infected with Penicillium italicum were determined. The MIC of LCEO against Penicillium italicum was 1.17 mg/mL. LCEO could significantly inhibit the mycelial growth and spore germination of Penicillium italicum, and the inhibition rates of mycelial growth and spore germination were (88.17 ± 0.30)% and (97.78 ± 0.30)% after 1 000 mg/L LCEO treatment, respectively. With an increase in LCEO concentration, the extracellular conductivity and extracellular pH of Penicillium italicum increased gradually, and the leakage of nucleic acid became more serious. SEM observation showed the mycelial linearity was lost, and irreversible morphological changes such as distortion, shrinkage and shriveling appeared on the mycelial surface after LCEO treatment. The higher the concentration of LCEO, the more serious the damage to the integrity and permeability of Penicillium italicum mycelia. The results of PI staining showed that the integrity of the cell membrane of the spores was damaged. In addition, the in vivo experiments showed that the disease incidence and lesion diameter of LCEO-treated citrus fruits decreased significantly compared to the untreated control group (P < 0.05), and the best effect was observed at 4 800 mg/L concentration. LCEO could effectively inhibit the spread of Penicillium italicum in citrus fruits, and it had a good control effect on citrus blue mold.