Chitosan is a natural product that has potential use in agriculture for managing diseases. Chitosan has been shown to effectively suppress storage rots when applied postharvest. Application of chitosan pre- and postharvest has potential to manage both latent and postharvest rots but these effects are not well studied. Furthermore, to determine the most effective strategy for using chitosan to manage apple diseases, research on application rates, chitosan molecular weight, phytotoxicity potential, and formulation is needed. The objectives of this study were to (1) identify non-phytotoxic concentrations of chitosan on apple fruit; (2) evaluate commercial chitosan products for reduction of postharvest disease severity on inoculated fruit; (3) evaluate the effect of pre-harvest chitosan applications on suppression of latent infections, postharvest rots, and fruit quality; and (4) evaluate the effect of pre-harvest plus postharvest chitosan applications on suppression of Penicillium expansum and Colletotrichum fioriniae on inoculated fruit. Under lab conditions, chitosan products applied at higher rates were more effective at reducing disease but tended to cause phytotoxicity. This phytotoxic effect was remediated when the product's pH was adjusted to ~5. Tidal Grow products applied at 1.0% (v/v) chitosan reduced lesion size caused by P. expansum and C. fioriniae on inoculated apples up to 86% compared to a water treatment. Pre-harvest applications of chitosan and Serenade ASO reduced bitter rot up to 85% on immature fruit in a research orchard. The results from this research suggest that Tidal Grow adjusted to pH ~5 can reduce postharvest diseases of apple fruit.
