Effects of chitosan treatment on avocado postharvest diseases and expression of phenylalanine ammonia-lyase, chitinase and lipoxygenase genes

Stem-end rot (Lasiodiplodia theobromae) and anthracnose (Colletotrichum gloeosporioides) are major postharvest diseases in avocado that cause severe postharvest losses throughout the supply chain. One of the strategies to reduce development of such decay agents resides in the application of resistance inducers (e.g. chitosan), capable of reducing fungal growth and inducing resistance in fruit tissues. The influence of chitosan treatment (1% or 1.5% w/v) was investigated on decay incidence, gene expression of phenylalanine ammonia-lyase (PAL), chitinase (CHI) and lipoxygenase (LOX) and antioxidant enzyme activity [superoxide dismutase (SOD) and catalase (CAT)] in i) drop-inoculated (without wounding) or ii) artificially infected (with wounding) avocado (cv. 'Hass') with L. theobromae or C. gloeosporioides pathogens and also in iii) naturally infected (natural inoculum) avocado. Fruit were dipped in 1% or 1.5% w/v chitosan dissolved in water for 3 min, followed by storage for 14 d and 28 d respectively at 7.5 degrees C, and thereafter for 5 d at 18 degrees C to simulate market shelf conditions. Chitosan at 1.5% significantly reduced the incidence of stem-end rot and anthracnose in both inoculated and naturally infected avocados. The up-regulation of PAL and down-regulation of LOX genes moderately allowed higher epicatechin contents (90 mg kg(-1) FW) in the exocarp, which could have contributed to improved anthracnose control. The up-regulation of CHI genes and higher SOD activity could have contributed to control of stem-end rot. Chitosan solution (1.5%) retained moderate levels of C7 sugars and firmness up to 5 d shelf life. The control of stem-end rot and anthracnose of avocados obtained with 1.5% chitosan can be ascribed to a combination of its antifungal and eliciting properties.