Host-pathogen interaction between spring wheat and Septoria nodorum Berk. with applications for wheat breeding were studied. Ultrastructure of interactions was studied using electron microscopic techniques. Following inoculation, conidia of S. nodorum germinate, form appressoria anda penetration peg which directly penetrates through the cell walls. It is suggested that most penetration attempts fail because of cellular defence reactions, formation of papillae and cell wall alterations.

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... on with low spore concentration reduced grain yield of Hankkija's Taava cultivar by 10 % and 1000-grain weight by 14 %. Inoculation with high spore concentration on large plots of Tähti cultivar reduced grain yield by 32 % and 1000-grain weight by 18 %. Inoculation with high spore concentration on normal breeding plots of Tähti cultivar reduced grain yield by 35 % and 1000-grain weight by 21 % and the grain yield of Kadett cultivar by 27 % and 1000-grain weight by 20 %. Inheritance studies on F2 progenies of spring wheat crosses involving susceptible and moderately or highly resistant parents suggest that heredity component of symptom expression is moderate level and breeding success depends mainly on efficient screening techniques. Resistance was associated with tallness in crosses, and cultivar trials suggest that resistance is positively associated with late maturation time. Field screening techniques based on small plots and artificial inoculation showed that the most resistant entries were wild Triticum species and late and tall cultivars. Seedling plant tests based on attached seedling leaves and detached leaves revealed easily the most resistant and most susceptible cultivars. The overall correlation between seedling tests and field tests was quite high. The results are discussed in relation to wheat breeding strategies for resistance to S. nodorum.