Multiple pre-harvest applications of antagonist Pseudomonas fluorescens ZX induce resistance against blue and green molds in postharvest citrus fruit

Abstract

The pre-harvest applications of antagonists are promising strategies to minimize various postharvest diseases. Given this, we sought to evaluate the capability of Pseudomonas fluorescens ZX in inducing resistance of postharvest citrus fruit against blue and green molds, caused by Penicillium italicum and Penicillium digitatum, respectively. Results showed that pre-treatment with P. fluorescens ZX significantly inhibited blue and green molds, generally providing stronger protection with increasing number of sprays. More specifically, three applications of the antagonist reduced disease incidence by about 47% and 41%, reduced lesion diameter by around 40% and 28%, and reduced disease index by approximately 69% and 57%, when exposed to blue and green molds, respectively. Mechanistically, these protective effects might be linked to the increased activities of superoxide dismutase, ascorbate peroxidase, peroxidase, and polyphenoloxidase, along with substantial response of phenolic acids (syringic, p-coumaric, ferulic, and sinapinic acids) and flavonoids (rutin, neohesperidin, hesperetin, sinensetin and tangeretin). P. fluorescens ZX survived well on fruit surface, and more importantly, pre-treatments exhibited no negative effects on fruit quality; on the contrary, it partly improved fruit quality during postharvest storage. Collectively, these results indicated that multiple pre-harvest applications of P. fluorescens ZX are useful for control of postharvest diseases.