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Environmental Influences on the Sensitivity of Fusarium oxysporum f. sp. lycopersici to Methylisothiocyanate. Klaus H. Domsch, Department of Botany and Plant Pathology, Oregon State University, Corvallis 97331, Present address of senior author: Institut für Bodenbiologie Forschungsanstalt für Landwirtschaft, Braunschweig-Völkenrode, Germany; Malcolm E. Corden, Department of Botany and Plant Pathology, Oregon State University, Corvallis 97331. Phytopathology 60:1347-1350. Accepted for publication 6 April 1970. DOI: 10.1094/Phyto-60-1347.

In shake culture, Fusarium oxysporum f. sp. lycopersici produces microconidia from phialides. A rapid decrease in pH of the culture medium from 6.0 to 4.2, followed by an almost equally rapid increase to about 8.0, is correlated with microconidia production. The pH increase is accompanied by decreased sensitivity to methylisothiocyanate (MIT). This decreased sensitivity to MIT is caused neither by the basic pH of the culture nor by morphologic changes in the spores as they age, but is associated with physiological maturity of the spores as demonstrated by the reduced biosynthetic activity of older spores. Reduced toxicity of MIT to Fusarium microconidia in potassium phosphate as compared to water at the same pH is not due to pH control during the assay, but is the result of phosphate ion interference with MIT toxicity.