Antifungal Activity of Plant Waste Extracts against Phytopathogenic Fungi: Allium sativum Peels Extract as a Promising Product Targeting the Fungal Plasma Membrane and Cell Wall

The harmful effect of synthetic fungicides on the environment and the development of resistance by fungi raises concerns about their security and future efficacy. In this work, we investigated plant by-products with the antifungal activity that could be safe alternatives to conventional fungicides. The in vitro antifungal potential of plant by-product extracts showed that garlic peel extract (GPE) was the most effective against several phytopathogenic fungi. Accordingly, in ex situ assays with apples, GPE significantly reduced the lesion size caused by subepidermal inoculation with Colletotrichum acutatum spores. In addition, Saccharomyces cerevisiae mutant strains affected in ergosterol synthesis showed higher resistance to GPE than the parental strain, indicating that the extract might target an intermediate of this pathway. Moreover, GPE affects the cell wall, given that bck1 and mkk1/mkk2 mutants were less able to cope with the stress because of the impairment of the remodeling mechanisms. Regarding the apoptosis-deficient mutant yca1, sensitivity was similar to that of the parental strain, suggesting that the extract does not induce apoptosis. A diverse group of sulfur compounds was identified by gas chromatography-mass spectrometry (GC/MS). Our findings contribute to the elucidation of the antifungal mechanism of GPE and highlight its potential as an alternative biofungicide in agriculture.