Antifungal activity of halogeno-salicylic acids against growth of Aspergillus niger

The relationship between the chemical structures of thirteen kinds of halogeno-salicylic acids and their antifungal activity against Aspergillus niger was examined by using the agar plate filter paper disc method at pH 3.7. The degrees of inhibition of fungal growth were estimated in terms of both initial strength of the activity (IS), which was obtained by measuring the diameter of the inhibition zone, and its durability in a plate culture of each sample. As a control test, the IS value of salicylic acid obtained at pH 3.7 with incubation for 3 days at 25° was defined as f=100. The durability was indicated in terms of the size of the final inhibition zone or the period required for the initial inhibition zone to disappear (below 6mm), under continuous incubation. In all examiniations, the presence of halogens (bromine, chlorine, and iodine) in the aromatic ring increased the antifungal activity. Among five kinds of monohalogeno-salicylic acids, the initial activity and the durability of the 3-chloro form were superior to those of other mono-chloro isomers, and the IS value of the 5-bromo form was higher than that of 4-bromo form. The IS value of the 3, 5-dichloro, dibromo, and diiodo forms were reduced in that order. The IS value of the 5-bromo-3-chloro form was as high as that of the 3-bromo-5-chloro form and was superior to that of dichloro form, although their duability was inferior to that of the 4, 5-dichloro form. In general, increasing the number of substituted halogen atoms resulted in higher antifungal activity. However, among all of the halogeno-salicylic acids tested, the 4, 5-dichloro form was more effective than the 3, 4, 5-tribromo or 4, 6-dibromo-3, 5-dichloro forms. In all tests, an inhibition zone of conidiogenesis was observed, and is shown as a p value (p=f1×(f1-f2)/100, f1; the inhibition factor of conidiogenesis, f2; the inhibition factor) in Table 3.