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Differential Effects of Sterol Inhibitors on Growth, Cell Membrane Permeability, and Ultrastructure of Two Target Fungi. H. Dahmen, Agricultural Division, Ciba-Geigy Ltd., 4002 Basle, Switzerland; H. C. Hoch(2), and T. Staub(3). (2)Department of Plant Pathology, New York State Agricultural Experiment Station, Cornell University, Geneva 14456; (3)Agricultural Division, Ciba-Geigy Ltd., 4002 Basle, Switzerland. Phytopathology 78:1033-1042. Accepted for publication 26 February 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/Phyto-78-1033.

The time course of effects of propiconazole on germ tube elongation and ultrastructure of Puccinia graminis f. sp. tritici were studied by using video time-lapse microscopy and electron microscopy. In addition, the effect of the fungicide on cell electrolyte leakage was determined. At 1 and 10 μg/ml, germ tube elongation was inhibited after a lag time of 15 and 20 min, although major changes in the cells’ ultrastructures were not noted nor was there a significant increase in electrolyte leakage from the cells. Many of the germ tube tips eventually ruptured 2–4 hr after initiation of the treatments. At 25 and 50 μg/ml, rapid cessation of germ tube elongation was accompanied by increased electrolyte leakage. Ultrastructural damage to cell membranes became apparent. The germ tube tips did not rupture at these fungicide concentrations. The effects noted for P. graminis were also confirmed with another fungus, Monilinia fructicola. The effects of the different sterol inhibitors fenpropimorph, imazalil, flutriafol, triadimenol, propiconazole, and penconazole on growth and on cell electrolyte leakage were compared. Penconazole caused a greater electrolyte leakage in M. fructicola than propiconazole, even though a five times high concentration was needed to observe an effect on mycelial growth. Of the other fungicides tested, only imazalil caused little electrolyte leakage, at concentrations as high as 50 μg/ml. The EC50 values for growth inhibition for M. fructicola ranged from 0.01 to 0.75 μg/ml for these fungicides. However, fungicides showed strong growth inhibition without any effects on electrolyte leakage. The results reported indicate that the direct action on fungal cell membranes may be a second mechanism of action for propiconazole and penconazole. Because this effect is only seen at concentrations substantially above those required for growth inhibition, it remains to be seen if it plays a role in practical disease control.