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Effect of the Lr9 Resistance Gene on Pathogenesis of the Wheat Leaf Rust Fungus. S. Ortelli, Swiss Federal Institute of Technology, Institut fur Pflanzenwissen-schaften, ETH-Zentrum, CH-8092 Zurich;. J. Nosberger, H. Winzeler, B. Keller, and M. Winzeler. Plant Dis. 80:14-18. Accepted for publication 10 July 1995. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-0014.

The pathogenesis of two leaf rust fungus isolates on seedlings of the susceptible cultivar Arina and two of its resistant near-isogenic lines (NILs) carrying Lr9 was studied by means of epifluorescence microscopy. The orientation of urediniospore germ tubes and differentiation of appressoria over stomata were similar on susceptible and resistant plants. The development of the fungus in the susceptiblc host was not stopped by any detectable host resistance reaction, and the number of haustorial mother cells increased exponentially. Fungal sporulation look place 7 to 8 days after inoculation. Host cells of the resislant Lr9 lines close to the haustorial mother cells died 24 to 44 h after inoculation due to a hypersensitive reaction. Before ceasing its growth, the fungus succeeded in forming generally only 1 to 3 haustorial mother cells. The necrotic area around an infection site reached an average area of 10,000 μm2. The potential influence of this defense reaction on yield is discussed.