Tannic acid exerts antifungal activity in vitro and in vivo against Alternaria alternata causing postharvest rot on apple fruit

Alternaria alternata is one of the main pathogens that causes black spot rot of apple fruit, mainly postharvest. Also, A. alternata can attack many other fruits and vegetables in most of the world's natural environments. In this study, the antifungal efficacy of tannic acid (TA) against A. alternata and its potential mechanisms were investigated. An in vitro bioassay suggested that 5.0 mg mL−1 TA significantly inhibited the mycelial growth and spore germination of A. alternata. The enzyme activities of hexokinase, phosphofructokinase and pyruvate kinase in glycolytic metabolism were decreased in A. alternata treated with TA. The accumulation of glucose content and the reduction of pyruvate content in A. alternata further confirmed that TA may block the glycolytic metabolism of this fungus, resulting in insufficient energy supply for cells. In addition, TA immersion treatment significantly reduced the development of lesion areas on apple fruits caused by A. alternata and delayed fruit softening during the storage period. The application of TA also increased the activity of chitinase, β-1,3-glucanase and phenylalanine ammonialyase and the contents of total phenolic compounds and flavonoids in apple fruit, indicating the activation of defense-related enzymes and phenylpropane metabolites. TA could potentially be used to prevent and control postharvest Alternaria rot by exerting antifungal activity on A. alternata and inducing resistance in apple fruit.