Genetic host resistance is the most economic and ecofriendly approach for plant disease control. Six commercial cultivars of wheat with different levels of adult plant resistance (APR) to brown rust were subjected to the genetic analysis in the present study. They were comprised of five Irani ('Mahdavi', Zarrin', 'Darab-2', 'Niknejat' and 'Atila-5') and an Australian cultivar ('Gascoyne'). They were crossed to a susceptible cultivar ('Roshan') and then backcrossed to either of the parents, and then P1, P2, F1, F2, BC1, BC2 generations (two crosses lacking BC1) were grown in a field nursery. The experiment was conducted at Gharakhil Research Station located in Mazandaran Province using a randomized complete block design. 'Bolani', a susceptible wheat cultivar was planted as spreader in the field margins and between the rows. Using the Modified Cobb Scale, average disease severity on the leaves was recorded when the susceptible cultivar had reached 100% severity. Results of analysis of variance for coefficient of infection (CI) showed significant differences among crosses as well as generations within crosses. The 34 entries belonging to 6 crosses showed wide ranges in CI and all generations within all crosses were significantly more resistant than the susceptible parent (P2). 'Darab-2' and 'Atila-5' were the most resistant cultivars to the brown rust. Estimated broad-sense and narrow-sense heritabilities of CI ranged from 0.60 to 0.81 and from 0.37 to 0.50, respectively. There was an agreement observed between individual scaling and joint scaling tests for the crosses having Iranian resistant parents (five out of six crosses) which in turn indicates the fitness of additive-dominance (AD) model for explaining the gene action in those crosses. Estimated genetic components of CI for leaf rust using AD model revealed that additive genetic effects play a major role in governing brown rust resistance in the crosses.
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