Possible Involvement of Reactive Oxygen Species in Action of Some Anti-Blast Fungicides

Abstract

A mode of action of three commercial fungicides against rice blast disease was studied using susceptible cultivar. Tricyclazole or fthalide when added to fungal nutrient medium altered melanin pigmentation of spores. The spores from treated cultures lost their pathogenicity. Simultaneously they acquired an enhanced sensitivity to inhibition of germination by leaf diffusates, intensive light, and superoxide-generating model system, perhaps due to decreased tolerance to active oxygen mediating these damages. Probenazole had no such action on fungus. In soil application of tricyclazole this substance (or its derivatives) liberated from leaf into infective drop in quantities sufficient, to alter pigmentation of a fungal mycelium and to prevent the disease. Treatment of rice plants with each of three fungicides also stimulated oxygen activation in leaf excretions. It was expressed in an overproduction of chemically detected superoxide and in correlative enhancement of the fungitoxicity mediated with O2- and H2O2. In infected plants these effects were observed only with kinds of fungicide application prevented the disease. It is concluded that the antioxidative capacity of a parasite is one of the conditions of its pathogenicity. Inhibition of this capacity and/or stimulation of oxygen activation in host may be additional causes of disease-controlling action of fungicides tested.