Antifungal activity of salicylic acid against Penicillium expansum and its possible mechanisms of action

Apple is a fruit widely produced and consumed around the world. Blue mold (Penicillium expansum) is one of the main postharvest diseases in apples, leading to a wide use of fungicides and the search for alternative products. The aim of this study was to assess the effect of salicylic acid (SA) against P. expansum, elucidating its mechanisms of action. The antimicrobial effect was determined by exposing conidia to a 2.5 mM SA solution for 0 to 120 min, followed by incubation. The effect of pH on the efficacy of SA against P. expansum was assessed both in vitro and in situ. The action mechanisms were investigated through fluorescence assays, measurement of protein leakage, lipid damage, and transmission electronic microscopy. SA was capable of inhibiting 90% of the fungal germination after 30 min, causing damage to the conidial plasma membrane and leading to protein leakage up to 3.2 μg of soluble protein per g of mycelium. The pH of the SA solution affected the antimicrobial activity of this secondary metabolite, which inhibited the germination of P. expansum and the blue mold incidence in apples in solutions with pH ≤ 3 by 100%, gradually losing its activity at higher pH.