Thymol controls the infection of Sclerotium sclerotiorum on Chinese cabbage

Sclerotium sclerotiorum can infect many vegetable species, which affects the quality of postharvest vegetables. Screening natural preservatives is an effective approach to control the infection of S. sclerotiorum. In this study, in vitro and in vivo inoculation experiments were conducted to investigate the effect of thymol (a plant-derived natural compound) on controlling the infection of S. sclerotiorum in Chinese cabbage. Thymol at different concentrations was added to the medium culturing S. sclerotiorum for in vitro experiment. Thymol inhibited mycelial growth in a dose-dependent manner. The fresh weight of S. sclerotiorum mycelia significantly decreased by 33.8%, 53.7%, and 88.1% upon thymol treatment at 20, 40, and 60 μg/mL, respectively. The dry weight of S. sclerotiorum mycelia decreased by 24.5%, 53.7%, and 82.9% upon thymol treatment at 20, 40, and 60 μg/mL, respectively.Thymol induced abnormality, break-down, and cell death of mycelia. The antifungal effect of thymol against S. sclerotiorum may result from cell membrane damage, supported by increases in mycelial relative conductivity and malonaldehyde content. Thymol also induced an increase in glycerol content in mycelia, indicating possible occurrence of osmotic stress. For in vivo experiment, S. sclerotiorum were inoculated on the leaves of Chinese cabbage followed by spraying thymol at 50 μg/mL. Thymol significantly inhibited the infection of S. sclerotiorum on the leaves, with infection lesion significantly decreased by 30.1% as compared to water treatment. In the leaves inoculated with S. sclerotiorum, thymol induced significant increase in the expression of two genes BrMPK3 and BrMPK4 with specific role against S. sclerotiorum infection. Chinese cabbage quality was also enhanced by thymol, evidenced by the increase in the content of chlorophyll, protein, reduced sugar, and vitamin C. These results indicated that thymol effectively controlled the infection of S. sclerotiorum on the leaves of Chinese cabbage by inhibiting fungal growth and inducing host defense. This study provides scientific basis for the development of natural preservatives for controlling postharvest infection of S. sclerotiorum.