Genetic analysis of partial rust resistance in faba beans

Abstract

Five parent populations of faba bean, differing in reaction to rust disease, were crossed to prepare a complete diallel of F₁ hybrids. Detached leaves of the parents and F₁ hybrids were challenged with 3 Australian rust populations and symptom development was monitored from 11 to 16 days after inoculation, using a disease score related to the percentage of leaf area covered with pustules, and a nominal value for pustule size. Additive gene action was very important in determining the response to rust disease, with ILB 3025 and ILB 3107 having similar low disease scores, Fiord and Acc 327 having high values, and Icarus an intermediate value. Cytoplasmic effects were also critical, with ILB 3025 having the most susceptible cytoplasm and ILB 3107 the most resistant, and this was most readily detected in terms of the size of the pustule. Heterosis conferred slightly lower disease scores and dominance was significant but of lesser importance than additive gene action, particularly for disease score. Rust population host gene action interactions were significant by the end of the observation period, but were much smaller than the main effects. The hybrid ILB 3107 ILB 3025 had less disease development than either parent, indicating the potential for developing more resistant cultivars using transgressive segregation. ILB 3107 shows particular promise for use as a female line as it had both nuclear and cytoplasmic factors to contribute to the development of resistant cultivars.