EFFECT OF THE LR9 RESISTANCE GENE ON PATHOGENESIS OF THE WHEAT LEAF RUST FUNGUS

The pathogenesis of two leaf rust fungus isolates on seedlings of the susceptible cultivar Arina and two of its resistant near-isogenic line s (NILs) carrying Lr9 was studied by means of epifluorescence microsco py. The orientation of urediniospore germ tubes and differentiation of appressoria over stomata were similar on susceptible and resistant pl ants. The development of the fungus in the susceptible host was not st opped by any detectable host resistance reaction, and the number of ha ustorial mother cells increased exponentially. Fungal sporulation took place 7 to 8 days after inoculation. Host cells of the resistant Lr9 lines close to the haustorial mother cells died 24 to 44 h after inocu lation due to a hypersensitive reaction. Before ceasing its growth, th e fungus succeeded in forming generally only 1 to 3 haustorial mother cells. The necrotic area around an infection site reached an average a rea of 10,000 mu m(2). The potential influence of this defense reactio n on yield is discussed.