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In this study, isolate SR99-UA (Race 70E 128) of wheat stripe rust (Puccinia striiformis West. f. sp. tritici Eriks) was used to infect susceptible spring wheat (cultivar Thatcher) and its near isogenic line possessing Yr-18 adult-plant resistance gene. Samples were taken at the flag leaf stage and observed using light, transmission and scanning electron micro-scopes. No major differences in the infection process were detected prior to haustorium formation, but sub-stomatal vesicles appeared to be formed after penetration, slightly ear-lier in susceptible than in the resistant genotypes. Higher numbers of haustoria were formed at any particular time in the susceptible cultivar than in the resistant near isog-enic line 2-3 days after inoculation. In the early stages of infection, host cell necrosis was not observed in the susceptible host, but was occasionally seen in the resistant isogenic line. Fourteen days after inoculation, the number of necrotic cells of hypersensitive re-sponse were higher on the resistant genotype than on the susceptible cultivar at this time when many of the infected mesophyll cells of the resistant host had collapsed. The first clearly developing pustules were usually observed 12 days after inoculation on the suscep-tible cv. Thatcher and 14 days after inoculation on its resistant near isogenic line. Eight-een days after inoculation, the mean number of pustules produced, per unit of infected leaf area, was high on the susceptible cultivar but on the resistant genotype the pustules were much less in number, diminutive and poorly sporulating.

Keywords: Wheat, Histological expression, Resistance, Stripe rust

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