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Reduced Infectability and Inoculum Production as Factors of Slow Mildewing in Knox Wheat. Gregory Shaner, Assistant Professor, Department of Botany and Plant Pathology, Purdue University, West Lafayette, Indiana 47907; Phytopathology 63:1307-1311. Accepted for publication 13 April 1973. DOI: 10.1094/Phyto-63-1307.

Powdery mildew development on ‘Knox’ and ‘Vermillion’ wheat was compared to determine the basis of Knox’s slow mildewing resistance. The upper three leaves of mature plants were inoculated with conidia of Erysiphe graminis. Fewer colonies developed on Knox than on Vermillion. The average colony size, and number of conidial chains per unit area of colony, were less on Knox than on Vermillion. The latter parameters were converted to a “sporulation index”, a measure of spore-producing capacity of a colony. The combined effect of reduced colony formation and reduced spore-producing capacity indicate that mildew should spread one-third as fast on Knox as on Vermillion. This agrees with differences in infection rate in the field.