Evaluating combinations of leather fungicides by inhibition zone

The synergism index (SI, Formula (1)) and graphical method (Fig. 1) expressed by the minimum inhibitory concentrations (MIC) are often used to evaluate the type of action of combined leather fungicides. The determination of MIC is a complicated and time-consuming process and the exact value is difficult to determine, so a rapid, simple method would be useful to evaluate synergistic combinations. In this study, a formula of the synergism index (SI*, Formula (3)) and the graphical method (Fig. 3) expressed by the diameters of inhibition zones were defined. (2-thiocyanoethyl)-cinnamate (A) and a commercial product of Isothiazoline whose active ingredients are the mixture 5-chloro-2-methyl-4-isothiazolin-3-one and 2-methyl-4-isothiazolin-3-one (B) were chosen for combination, and the diameters of inhibition zones and minimum inhibitory concentrations for the effectiveness of different combinations to nine fungi were determined to verify the feasibility of the two methods. Results show that the values of SI calculated by MIC and the value of SI* calculated by diameters of inhibition zones are only slightly different, but they both show the same tendency in that, to most fungi and for most proportions, the values of SI and SI* are all less than 1.0. So, the different combinations of A and B show a synergistic effect or additive effect and the method of SI* is viable to evaluate their action. The relation between the value of SI* and the type of action is same as that of SI. Regarding the graphical method, the results for MIC show a dramatic synergistic effect that is different from that of SI method, and the results of evaluation of diameters of inhibition zones is similar to that of SI* method. So the graphical method expressed by the diameters of inhibition zones is also usable. As the determination of inhibition zones is a relative simple process, the two new methods are very convenient for evaluating synergistic combinations of leather fungicides.