The fungitoxicity of chlorophenols to the pathogenic fungi, Fusarium oxysporum and Rhizoctonia solani: A structure‐activity relationship study

Inhibition of hyphal growth of Fusarium oxysporum ^f. sp. melonis and Rhizoctonia solani by a series of chlorinated phenols was studied. Excluding the 2‐chloro derivative, R. solani was found to be more sensitive to chlorophenols than F. oxysporum. When the octanol‐water partition coefficients (expressed as π values) of the compounds were plotted against the effective median dose (ED50), a positive and linear relationship was disclosed. Fungitoxicity was enhanced with the increase in the number of chlorine substituents and their position on the phenol ring was an important factor in determining the biological activity. Toxicity was related to the undissociated (hydrophobic) state of the molecule. The toxicity of the chlorinated phenols varied with the pH of the medium: the relationship of this to the pK_a of the compound indicated that permeation of the toxicant through the fungal cell walls and plasma membranes increased with increasing concentration of nonionized molecules. The different inhibition pattern of pentachlorophenol is discussed in light of its property as a lipophilic ion.