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Survival of Endoconidia and Chlamydospores of Thielaviopsis basicola as Affected by Volatile Soil Fungicides. G. C. Papavizas, Plant Science Research Division, ARS, USDA, Beltsville, Maryland 20705; J. A. Lewis, Plant Science Research Division, ARS, USDA, Beltsville, Maryland 20705. Phytopathology 62:417-422. Accepted for publication 2 November 1971. DOI: 10.1094/Phyto-62-417.

Methylisothiocyanate (MIT) and volatile materials from the decomposition of 3,5-dimethyl-tetrahydro-1, 3,5,2H-thiadiazine-2-thione (DMTT) and sodium N-methyldithiocarbamate (SMDC) in soil reduced germinability of endoconidia and chlamydospores of Thielaviopsis basicola. Fungicides with low vapor pressures in aqueous solutions or in suspensions did not produce vapors in soil toxic to spores of T. basicola. The toxicity of the vapors of MIT, DMTT, and SMDC to the propagules in soil was enhanced by increased concentrations and length of exposure. SMDC was more effective than MIT and DMTT. Endoconidia of T. basicola were slightly more sensitive than chlamydospores to the vapors from SMDC. The three volatile fungicides were fungicidal rather than fungistatic. Alfalfa hay, kaolinite, and NH4Cl, added to soil with DMTT, reduced the effectiveness of the fungicide. Alfalfa hay added to soil 3 and 9 weeks before the fumigant, and montmorillonite added with the fumigant, did not reduce the effectiveness of the fungicide. Thielaviopsis basicola inoculum in undecomposed or partially decomposed bean hypocotyls was less sensitive to vapors from DMTT than was free chlamydospore inoculum in soil. The effectiveness of DMTT to reduce germinability of chlamydospores and black root rot of bean varied with the inoculum density of the pathogen in soil.