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Metabolic Effects Related to Fungitoxicity of Carboxin. Nancy N. Ragsdale, Faculty Research Assistant, Department of Botany, University of Maryland, College Park 20742; H. D. Sisler, Professor, Department of Botany, University of Maryland, College Park 20742. Phytopathology 60:1422-1427. Accepted for publication 14 April 1970. DOI: 10.1094/Phyto-60-1422.

Concentrations of carboxin (5,6-dihydro-2-methyl-1,4-oxathiin-3-carboxanilide) which reduce the rate of growth of Rhizoctonia solani and Verticillium albo-atrum about 90% inhibit glucose oxidation about 50%. Glucose oxidation by conidia of Neurospora crassa and mature sporidia of Ustilago maydis is unaffected or only slightly inhibited, whereas acetate oxidation by the two organisms is inhibited 70-90%. Carboxin is about 10 times as toxic to growth of U. maydis, N. crassa, and Saccharomyces pastorianus on an acetate substrate as on a glucose substrate. The fungicide is detoxified by riboflavin or riboflavin phosphate in the presence of light. Syntheses of ribonucleic acid, deoxyribonucleic acid, and protein are reduced 60-90% in rapidly metabolizing cells of R. solani, U. maydis, V. alboatrum, and N. crassa. In sporidia of U. maydis, the high energy nucleotide phosphate is reduced 64%; but the level of free nucleotides is reduced only 21%. Interference with energy generation or with the synthetic aspects of the citric acid cycle appears to be primarily responsible for carboxin toxicity.