Leaf and stem resistance to gummy stem blight [Didymella bryoniae (Auersw.) Rehm.] in five resistant by susceptible crosses of cucumber (Cucumis sativus L.) was investigated using generation means analysis. No single gene of major effect controls either leaf or stem resistance to gummy stem blight in these five crosses. The mean number of effective factors controlling leaf resistance in the cross `Slice' × `Wis. SMR 18' was estimated to be at least five. Estimates of broad- and narrow-sense heritabilities indicated that environmental effects were larger than genetic effects. In general, additive variance was the larger component of genetic variance. Epistasis was significant in most crosses, and dominance was present in several crosses. Additive gene effects contributed more to resistance than to susceptibility in contrast with dominance gene effects. Reciprocal differences for leaf rating were detected in the crosses M 17 × `Wis. SMR 18' and `Slice' × `Wis. SMR 18'. Phenotypic correlations between leaf and stem ratings were moderate (r = 0.52 to 0.72). Estimates of genetic gain for resistance to gummy stem blight ranged from low to moderate. Breeding methods that make best use of additive variance should be used because much of the variance for resistance is additive, and dominance effects, at least in these crosses, tended to contribute to susceptibility.
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