Following diallel crosses among the cultivars Boyne, Meeker, Malling Jewel, Glen Prosen and Glen Clova, the progenies and clonally propagated parents were exposed initially in a plantation (and assessed at the telial stage) and the following spring were inoculated with urediniospores in a greenhouse (and assessed 13 and 18 days after inoculation). Boyne was completely resistant, developing only chlorotic flecking on inoculated leaves in the greenhouse and no sporulation. It is postulated that this reaction was determined by a major gene which is designated Yr. Some of the other parents showed slow rusting, and selfed progeny of Boyne that were susceptible also showed low levels of rust, suggesting that Boyne carries both major and minor-gene resistance. Diallel analyses of both the field and greenhouse data from the susceptible segregates indicated that the degree of incomplete resistance present was determined predominantly by additive gene action, though small but significant nonadditive effects also occurred. Meeker was the most resistant parent (slow rusting type) and contributed the most resistance to the progenies. The Meeker progenies also segregated for gene H (cane hairiness), the hh genotype being the more resistant.