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Relationships of Molecular Structure of 1,4-Oxathiin Fungicides to Chemotherapeutic Activity Against Rust and Smut Fungi in Grasses. John R. Hardison, Plant Pathologist, Crops Research Division, ARS, USDA, and Department of Botany and Plant Pathology, Oregon Agricultural Experiment Station, Corvallis 97331; Phytopathology 61:731-735. Accepted for publication 29 January 1971. DOI: 10.1094/Phyto-61-731.

Twelve chemicals, including 5,6-Dihydro-2-methyl-1,4-oxathiin-3-carboxanilide (carboxin), its dioxide (sulfone) form (oxycarboxin), and 10 other substituted analogs of carboxin, from UniRoyal Chemical Corp., Bethany, Conn., were evaluated for systemic fungicidal activity against stripe rust, Puccinia striiformis; stripe smut, Ustilago striiformis; and flag smut, Urocystis agropyri; after soil application and root uptake in Poa pratensis. Activity of carboxin was poor on rust, fair on flag smut, and poor on stripe smut. Oxycarboxin was excellent on rust and good on stripe smut; it eradicated U. agropyri. The monoxide form (F831) was superior to carboxin for rust but inferior to oxycarboxin for all three diseases. Oxycarboxin plus a 4-methoxy substitution to the phenyl ring (F837) had poor activity against U. striiformis and reduced activity against U. agropyri, but maintained good rust control as compared to oxycarboxin. Serious reduction or loss of activity against all three diseases was apparent in most nonoxidized analogs with substitutions to the phenyl ring, including: 2-phenyl (F427), 2,6-diethyl (F829), 2-methyl, 3-chloro (F861), and 4-methoxy (F934). A 2,3-dimethyl derivative (F827) with a nonoxidized heterocycle impaired rust control but had much better activity against U. striiformis and U. agropyri than carboxin and less phytotoxicity than oxycarboxin. F872, which combines active substitutions of F827 and oxycarboxin, maintained strong activity against all three diseases with somewhat less plant injury.