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Fungitoxicity and Structure-Activity Relationships of Some Oxathiin and Thiazole Derivatives. Marten Snel, Graduate Student, Department of Botany, University of Guelph, Guelph, Ontario; B. von Schmeling(2), and L. V. Edgington(3). (2)Agricultural Chemicals Research, UniRoyal Chemical, Division of UniRoyal, Inc., Bethany, Connecticut 06525; and (3)Professor, Department of Botany, University of Guelph, Guelph, Ontario. Phytopathology 60:1164-1169. Accepted for publication 3 March 1970. DOI: 10.1094/Phyto-60-1164.

ed₅₀ values of carboxin (5,6-dihydro-2-methyl-1, 4-oxathiin-3-carboxanilide), its sulfone analog, oxycarboxin (both systemic fungicides used for control of diseases caused by Basidiomycetes), and 13 related, substituted oxathiin and thiazole compounds were determined to a selected number of Basidiomycetes, Deuteromycetes, and a Zygomycete. Eradicant activity of these compounds was determined against bean rust, Uromyces phaseoli typica. Substitutions in the carboxin molecule studied do not increase the spectrum of fungi to which the oxathiins are toxic. A number of yeastlike lower Basidiomycetes belonging to the Tremellales (jelly fungi) proved to be insensitive to oxathiins. The only Deuteromycete in this study sensitive to oxathiins was Monilia cinerea f. americana. The 3’-methyl-analog of carboxin is the only compound surpassing the fungitoxicity of carboxin. Electron withdrawing groups (–Cl and –NO₂) substituted in the aniline ring markedly reduce fungitoxicity. Replacement of the 2-methyl-oxathiin moiety by an o-tolyl,2,4-dimethylthiazolyl, 2-amino-4-methyl-thiazolyl, or even to some extent by a butyryl group, results in compounds retaining the original biological activity. Benzanilide is significantly less toxic to Rhizoctonia solani than o-toluanilide, indicating that a methyl group in position 2 is necessary for good toxicity. Results of evaluation of the eradicant activity of oxathiins against bean rust correlate very well with those of in vitro fungitoxicity tests, suggesting that oxathiin systemic fungicides act by virtue of their fungitoxicity, rather than by altering host metabolism.