The effect of hydroxyl radical scavengers dimethyl sulphoxide, p-nitrosodimethylaniline, ethanol, benzoate, salicylate and thiourea was studied on sclerotial differentiation and growth of Sclerotinia sclerotiorum and Rhizoctonia solani. There is a correlation between both scavenger concentration and scavenger-hydroxyl radical reaction rate with delay and inhibition of differentiation at growth-noninhibiting scavenger concentrations. Growth-inhibiting scavenger concentrations further increased delay and inhibition of differentiation, and eventually stopped fungal growth, acting as antifungal antioxidant alternatives to traditional fungicides. p-nitrosodimethylaniline (with the highest hydroxyl radical reaction rate) was the most effective inhibitor of sclerotial differentiation and growth. The results strongly support the theory that oxygen free radicals induce differentiation in sclerotium-producing phytopathogenic fungi.