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Mode of Action of Oxathiin Systemic Fungicides. I. Effect of Carboxin and Oxycarboxin on the General Metabolism of Several Basidiomycetes. D. E. Mathre, Associate Professor, Department of Botany and Microbiology, Montana State University, Bozeman 59715. Phytopathology 60:671-676. Accepted for publication 7 November 1969. DOI: 10.1094/Phyto-60-671.

Carboxin (5,6-dihydro-2-methyl-1,4-oxathiin-3-car-boxanilide) and oxycarboxin (5,6-dihydro-2-methyl-1, 4-oxathiin-3-carboxanilide-4,4-dioxide) at concentrations inhibitory to growth also inhibited the respiration of the sensitive Basidiomycetes Rhizoctonia solani, Ustilago nuda, and Ustilago maydis. When glucose-U-¹⁴C was administered to these organisms, both fungicides caused an increased release of ¹⁴CO₂ when compared to a control. Time course studies with glucose-1-¹⁴C or glucose-6-¹⁴C indicated that there was an initial inhibition of appearance of ¹⁴CO₂ from carboxin-treated cells as compared with nontreated cells, but after 1 hr there was a greater appearance of ¹⁴CO₂ from the carboxin-treated cells as compared with nontreated cells, suggesting that a block in the metabolism of pyruvate causes a recycling of carbon through pathways of glucose metabolism. The appearance of ¹⁴CO₂ when carboxin-treated cells were administered pyruvate-2-¹⁴C, acetate-1-¹⁴C, or acetate-2-¹⁴C was inhibited 75-95% compared with a control. A buildup of label in succinate with a decreased amount of label in citrate, fumarate, and malate occurred in carboxin-treated cells that were administered glucose-U-¹⁴C, acetate-1-¹⁴C, or acetate-2-¹⁴C. Carboxin inhibited the incorporation of phenylalanine into protein about 30% and the incorporation of uracil into RNA about 75%.