Ultrastructural changes induced by the systemic fungicides triadimefon, nuarimol and imazalil nitrate in sporidia ofUstilago avenae

The effects of the systemic fungicides triadimefon, nuarimol, and imazalil nitrate (5 μg ml-1 for 8 and 16 h) on the ultrastructure of sporidia of Ustilago avenae were studied using transmission and scanning electron microscopes. Thin-section and freeze-fracture techniques demonstrated fine structural alterations to the cytology of the fungus. Compared with untreated controls, in which the cells were generally unicellular and uniform in shape and ultrastructure, fungicide-treated cells exhibited considerable thickening of peripheral cell walls and septa, an increased number of mitochondria that were enlarged and irregular in shape, greater amounts of endoplasmic reticulum, and extensive vacuolisation and accumulation of lipid bodies. The vacuoles were filled with membrane-like aggregates and vesicles, which could be extruded by exocytosis. The plasmalemma ultrastructure of fungicide-treated sporidia, observed by scanning electron microscopy after freeze-fracture was significantly different from that of control cells. The antifungal agents caused hemispherical pits, protrusions, invaginations, and aggregation of intramembrane particles on the membrane fracture faces. Specific structures, on the plasmalemma of plasmatic and extraplasmatic fracture faces, support the assumption of exocytosis. The ultrastructural changes were more pronounced after triadimefon and nuarimol treatment than after imazalil nitrate treatment. An extended incubation time (16 h) with triadimefon and nuarimol resulted in complete destruction of the cellular system of most sporidia.