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Quantitative Inheritance of Fungicide Tolerance in a Natural Population of Cochliobolus carbonum. D. R. MacKenzie, Former Research Assistant, The Pennsylvania State University, University Park 16802, Present address; CIMMYT, Londres 40, Mexico 6, D.F., Mexico; R. R. Nelson(2), and H. Cole(3). (2)(3)Professor of Plant Pathology, and Associate Professor of Plant Pathology and Chemical Pesticides, The Pennsylvania State University, University Park 16802. Phytopathology 61:471-475. Accepted for publication 15 November 1970. DOI: 10.1094/Phyto-61-471.

A complex, heritable system for fungicide tolerance in a natural population of Cochliobolus carbonum was identified by sexual progeny response to selected concentrations of thiram as measured by vegetative growth rates on solid medium. The system is not associated with innate vegetative vigor, since fast growers on medium without fungicide were not necessarily tolerant to the fungicide. Tolerance to thiram was not associated with tolerance to captan or zineb. Random phenotypic responses to thiram for five vegetative transfers were suggestive of a cytoplasmic or nuclear-cytoplasmic system. The data suggest a re-evaluation of past interpretations of dose response curves, since phenotypic expression would vary with inheritance as well as with the environmental stress. The importance of this type of heritable system of fungicide tolerance under fungicidal spray programs in a natural disease situation is not known.

Additional keywords: genetics, maize, Helminthosporium carbonum.