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Infection Structure Differentiation by Wheat Stem Rust Uredospores in Suspension. Larry D. Dunkle, Research Assistant, Department of Botany, University of Wisconsin, Madison 53706, Present address of senior author: Department of Plant Pathology, University of Nebraska, Lincoln 68503; Paul J. Allen, Professor, Department of Botany, University of Wisconsin, Madison 53706. Phytopathology 61:649-652. Accepted for publication 4 January 1971. DOI: 10.1094/Phyto-61-649.

A procedure is described for the induction of differentiation of infection structures (appressorium, vesicle, and infection hypha) by wheat stem rust uredospores suspended in a Tween-buffer-nonyl alcohol medium. Critical factors involved in infection structure formation, induced by a heat shock of 30 C, include the spore:liquid ratio, the duration of heat shock, and the composition of the suspending medium present during the induction period. Endogenous metabolites which are apparently different from the germination inhibitor and volatile stimulant influence the progress of differentiation.