Inhibition efficiency of natamycin pretreatment before harvest on gray mold of post-harvest grapes

Abstract: Gray mould caused by Botrytis Cinerea is one of the most harmful diseases to grape production and has become an obstacle to grape storage. Natamycin is a natural antimycotic polyene commonly used to prevent yeast and mould contamination of daily food. However, there is no conclusive report on the possible effect of this antibiotic on post-harvest gray mold for grapes. In this paper, the effects of natamycin on grape Botrytis Cinerea were studied through artificially inoculated experiments and related bioassay. Inhibition potentials on post-harvest gray mold for grapes were further studied through pre-harvest dipping treatments and related physiological analysis. Results showed that, at both room temperature and low temperature conditions, growth of Botrytis Cinerea inoculated on fruit surfaces was obviously inhibited by 500-mg/L natamycin antimicrobial. The inhibition rate of the 'Kyoho' and 'Muscat' grape reached 95.23% and 87.84%, respectively. The inhibition ratio under low temperature was higher than that at room temperature, which achieved highly significant levels (p<0.01). The microscopic morphology of mycelium, terminal growing points and spores were also changed into unfavorable growth. Mycelium edges became rough, separation was not clear, terminal growing points wilted, and spores became malformed. For both of the two varieties, mildew rate, abscission rate, and weight loss rate of pre-harvest dipping treatment were lower than that of post-harvest dipping treatment. Fruit hardness of pre-harvest dipping treatment was higher after being stored for 80 days. The increase of the Botrytis Cinerea population on fruit surfaces was significantly suppressed by the pre-harvest dipping treatment of 500 mg/L natamycin antimicrobial. Compared to control, Botrytis Cinerea population on 'Kyoho' and 'Muscat' grape fruits were reduced by 73.66% and 78.71% respectively after pre-harvest dipping treatment of natamycin at the end of storage. The mildew rate, abscission rate of fruit, and browning index of stem were also reduced during storage period. Compared to control, the mildew rate of 'Kyoho' and 'Muscat' were reduced by 40.77% and 50.36%, the abscission rate of fruit were reduced by 43.93% and 43.20%, and the browning index of stem were reduced by 50.86% and 48% respectively when stored for 80 days. In addition, the ethylene (ETH) generation rate of grape clusters was inhibited by pre-harvest dipping treatment of 500 mg/L natamycin antimicrobial, and the activity of phenylalnine ammonialyase (PAL), polyphenol oxidase (PPO), and peroxidase (POD) in the stem were reduced to some extent. Chitin and β-1, 3-glucan are main structural components of fungal cell walls. Chitinases and β-1, 3-glucanases produced in plant tissue have the abilities to degrade Chitin and β-1, 3-glucan, which will help eliminate pathogenic fungus. Furthermore, activity of chitinases and β-1, 3-glucanases in grape fruit were largely induced by natamycin treatment, directly or indirectly, so as to increase the resistance to postharvest grey mould. In conclusion, pre-harvest treatment of natamycin can greatly help control grey mould and improve the quality of table grapes during storage. Findings of the present study would be helpful in designing management measures to reduce incidence and severity of mildew, abscission, and browning of different grape varieties. The findings also demonstrated the potential for using natamycin in controlling natural infections of grape postharvest pathogens. These findings can be employed by grape suppliers to provide qualitatively and quantitatively better produce to the local and/or export markets and release great application potential in the field of keeping table grapes fresh.