Inhibition of TTC reduction of the mycelial homogenates of the rice-blast fungus by fungicides.

Abstract

A study has been made on the TTC reduction of the mycelial homogenate of rice-blast fungus, Piricularia oryzae Cav., particulaurly on its inhibition by several fungicides. The mycelia grown on Tochinai's synthetic media containing potato decoction at 27°C for 72 hours in shaken culture, were homogenized and fractionated according to the scheme shown in fig. 1, and the supernatant obtained was poured into Thunberg tube for test. After 2 hours of incubation at 28°C, the produced TPF (triphenyl formazan) was measured by an electrophotometer (filter S 48). As a relatively marked TTC reduction of the homogenates was noted at a rate of 20g mycelia per 100ml homogeaate, this concentration was used throughout the experiments.A considerable portion of the activity of TTC reduction was retained in the supernatant fraction after the centrifugation of mycelial homogenates (Table 3). TTC reductivity of the homogenate was lost by heat treatment at 50°C for 10 minutes, and by addition of malonate at a concentration of 0.02M, while reduction was accelerated by adding 0.02M succinate (Tables 4 and 5). Therfore it seems to be reasonable to assume that the TTC reduction above mentioned is probably attributable to succinodehydrogenase system of the present fungus. As shown in the figures 2 and 3, TTC reduction was retarded by various fungicides, especially by those belonging to mercuric group, among which MEMC and EMP exhibited a strong inhibition at concentrations of 10-5-10-7M. The grade of inhibition decreased in the following order; PMA, MC 10-4-10-6M; PMTS, EMTS 10-4-10-5 M; copper sulfate, thiram and zineb 10-3-10-5 M; DNPTC, TCDNB 10-3-10-4M; PCNB did not inhibit at all. These results are consistent with those obtained in the author's another experiment on the vapor action of fungicides and the direct fungitoxicity to spore germination. Cysteine at concentrations of 10-2-10-4M increased the rate of TTC reduction, and reversed the inhibition by the mercuric compounds such as PMA, MC and EMP.