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Deviation from the Regression of Infection on Heading and Height as a Measure of Resistance to Septoria tritici Blotch in Wheat. L. T. Van Beuningen, Wheat Pathologist, Wheat Project for Warm Marginal Areas, International Maize and Wheat Improvement Centre (CIMMYT), C.C. 1170, Asuncion, Paraguay. M. M. Kohli, Breeder/Representative, Wheat Project for Warm Marginal Areas, International Maize and Wheat Improvement Centre (CIMMYT), C.C. 1170, Asuncion, Paraguay. Plant Dis. 74:488-493. Accepted for publication 3 January 1990. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/PD-74-0488.

Data acquired over three years (19841986) on Septoria tritici blotch reported from the Advanced Lines of the Southern Cone (LACOS) wheat nursery, were analyzed and an average relative coefficient of infection was calculated for each entry in each year. In all three years, a significant negative correlation was found between Septoria tritici blotch and days to heading and height, which accounted for nearly half the variance in infection by Septoria tritici. A resistance parameter, deviation from the regression of the infection on heading and height, was defined by eliminating the linear effect of escape mechanisms through late heading or tall stature. This parameter is considered the best measure of true genetic resistance, which is of most interest to breeders. It facilitated identification of the most resistant germ plasm, using disease notes taken at a single critical moment. Germ plasm that originated in areas prone to epidemics by S. tritici, all of which are under strong maritime influence or are in the Parana River Basin, generally had higher levels of true resistance as well as escape mechanisms, resulting in reduced field infection. For breeding value, however, some earlier, shorter lines showing intermediate infection levels are thought to be superior.

Keyword(s): Triticum aestivum, Mycosphaerella graminicola, South America.