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Influence of Plant Age on the Expression of Slow-Mildewing Resistance in Wheat. G. D. Gustafson, Former graduate research assistant, Department of Botany and Plant Pathology, Purdue University, West Lafayette, IN 47907, Present address of senior author: Biophysics Laboratory, University of Wisconsin, Madison 53706; Gregory Shaner, professor, Department of Botany and Plant Pathology, Purdue University, West Lafayette, IN 47907. Phytopathology 72:746-749. Accepted for publication 5 October 1981. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-746.

Wheat cultivars Monon and Knox and Purdue breeding lines P6217, P65104, P6693A7, and P6693A12 were inoculated in the greenhouse with Erysiphe graminis f. sp. tritici at 12 growth stages. In one set of experiments, disease severity was assessed on the upper three leaves. The sporulation index, a measure of the average spore-producing capacity of a population of colonies, was calculated for the upper two leaves. In a second set of experiments, disease severity, sporulation index, and colony density per square millimeter of leaf were determined on the penultimate leaf (F-1). For a given cultivar or breeding line, the average colony size was nearly constant at all growth stages; however, disease severity and sporulation index varied with growth stage at time of inoculation and with the leaf inoculated. Resistant and susceptible cultivars could be distinguished by colony size when plants were inoculated at any growth stage after GS 37 (flag leaf just visible). However, on the basis of disease severity or sporulation index, resistant and susceptible cultivars were most easily distinguished when plants were inoculated between GS 43 (boot just beginning to swell) and GS 64 (anthesis half-completed) on F-1 or between GS 40 (flag leaf collar just visible) and GS 58 (inflorescence completely emerged) on F-2, the leaf below F-1. Resistant and susceptible cultivars could not be distinguished by disease severity on flag leaves.

Additional keywords: general resistance, powdery mildew, Triticum aestivum.