Fungal pathogens identification of postharvest apples in bagging-free cultivation pattern and antifungal activity of essential oil

Abstract: 【Objective】Apple occupies a dominant position in fruit production of China. Growing inter-ests exist in examining the potential of non-bagging-fruit cultivation pattern because of the decline of in-ternal quality of bagging-apple fruits and the increase of labor cost as well as the environmental pollution.It is recognized that apple-bagging free cultivation is an inevitable trend in the development of China’s ap-ple industry. However, with non-bagging-fruit cultivation pattern, some problems still occur such as se-rious pathogens and insects. In order to identify pathogens types and illuminate occurrence regularity of postharvest fungal diseases of non-bagging apples, we aimed to investigate the incidence of postharvest decay of bagging and non-bagging apple fruits in the main apple production areas of Shandong Province, and evaluate the efficacy of essential oils as a green antibacterial method.【Methods】Traditional morphological identification method combined with ITS analysis were performed for identification of the pathogenic fungi strains isolated from rotten non-bagging apples. Then, based on the previous re-sults, we optimally selected cinnamon essential oil to assess its antifungal activity by in vitro and in vivo fumigation against such strains. Then, antifungal mechanism of cinnamon essential oil was assessed through determining the main chemical components by gas chromatography/mass spectrometry (GC/MS) analysis, and by observing spore germination, configuration of mycelia cells and ultrastructural al-terations through scanning electron microscope (SEM) and transmission electron microscopy (TEM), re-spectively.【Results】The postharvest infection incidence of non-bagging apples was significantly high-er than that of the bagging apples (p<0.05). Six strains were isolated and identified as Botryosphaeria dothidea, Alternaria, Trichothecium roseum, Pestalotiopsis vismiae, Colletotrichum gloeosporioides and Acremonium sclerotigenum according to the analysis on morphology and molecular biology. The results of GC/MS analysis showed that there were two major kinds of compounds in cinnamon essential oil we used, respectively, benzaldehyde (4.577 min) and trans-cinnamaldehyde (9.229 min), and the lat-eraccounted for 69.5% . Cinnamon essential oil exhibited obvious antifungal effect on all of the six strains, and the inhibitory rates could reach to 80% at 80 μL·L^-1 concentration and 100% at 100 μL·L^-1 concentration except for C. gloeosporioides. For T. roseum and Alternaria, the inhibitory rates of cinna-mon essential oil were more than 90% at 20 μL·L^-1 concentration. Based on the radial growth of myceli-um on the medium, the fumgitoxic ability of cinnamon essential oil was observed in a dose-dependent manner, which was not as dramatic as that under in vivo conditions. In the concentration range of 20-80μL·L-1, with the increase of concentration, the decay rates showed a downward trend. The decay rate of 80 μL·L^-1 treatment decreased by 33% compared with the control, while at 100 μL·L^-1, the decay rate was increased significantly compared with that at 80 μL·L^-1 (p < 0.05), and we speculatde that the ex-cessively high concentration of cinnamon essential oil would cause certain phytotoxicity to the fruits. We choosed B. dothidea, which is a serious hazard for bagging-free apples before and after harvesting, to analyze the effects of cinnamon essential oil treatments on its spore germination, hyphae morphology and cell internal structures. Cinnamon essential oil effectively inhibited the germination of B. dothidea spores and the elongation of thebud tubes. The germination rate of spores with 400 μL·L^-1 treatment de-creased to 6%, and the length of the bud tube was reduced by 94.51% at 600 μL·L^-1 compared with the control group. SEM of B. dothidea exposed to cinnamon essential oil showed irreversible deleterious morphological such as lack loss of integrity and rigidity of the cell wall, folding of the cell, and shrink in appearance on the surface of mycelium compared to those of the control group.【Conclusions】In the non-bagging cultivation mode, the incidence of postharvest pathogenic fungi was significantly higher than that of bagging apples. Cinnamon essential oil exhibited excellent antibacterial effect both in vitro and in vivo, and was proved to be a promising candidate as safe and effective antifungal agent for post- harvest fungal spoilage of non-bagging apples. It can be considered to further study the comprehensive control efficiency of the cinnamonon the diseases during the postharvest storage of bagging-free apples combining with cloves or other essential oils. In practical applications, it is recommended to innovate the load form or embedding technology to further improve the stability of the essential oils.