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S U M M A R YThe inheritance of resistance in red raspberry (Rubus idaeus) to yellow rust (Phragmidium rubi-idaei) was studied in a diallel cross among the cultivars Boyne, Meeker, Malling Jewel, Glen Prosen and Glen Clova. The progenies and clonally propagated parents were exposed initially in a rust-infected plantation where the incidence and severity of infection was assessed at the telial stage. The following spring leaves on new canes of the same plants were inoculated with urediniospores in a glasshouse at 18 f 2 "C. The latent period and number of uredinia per cm2 of abaxial leaf surface were determined 13 and 18 days after inoculation. Cv. Boyne developed -only chlorotic flecking on inoculated leaves in the glasshouse and no sporulation was observed. It is postulated that this reaction was determined by a major gene which is designated Yr. Some of the other parents showed incomplete resistance of the slow rusting type, and segregants of Boyne selfed that were susceptible also showed low levels of rust, which suggests that Boyne carries both major-and minor-gene resistance. Diallel analyses of both the field and glasshouse data from the susceptible segregates indicated that the degree of incomplete resistance present was determined predominantly by additive gene action, though small but significant non-additive effects also occurred : cv. Meeker was the most resistant parent and contributed the most resistance to the progenies.The Meeker progenies also segregated for gene H , which determines cane hairiness and is known to be associated with resistance to other diseases. Segregants without hairs (genotypes hh) were found to be the more resistant to rust.Phragmidium rubi-idaei (DC.) Karst. is endemic in Britain on the wild red raspberry (Rubus idaeus L.). It has occurred sporadically in commercial raspberry plantations during the past 50 years but was not considered serious (Moore, 1944;Foister, 1961) until in 1977 several severe outbreaks occurred in the North of Scotland (Wale, 1981); since then it has become widespread throughout Britain on cvs Malling Delight and Glen Clova.This rust occurs throughout raspberry growing areas world-wide except in

Phragmidium rubi‐idaei produced five spore stages on red raspberry and overwintered as teliospores adhering to canes. In the field spermagonia appeared in a single flush in early summer on leaves of the primocanes (first‐year canes) and on leaves of the lateral shoots of fruiting canes, and were followed by aecia, uredinia and telia. Aecia and uredinia occasionally appeared on petioles and sepals and uredinia on drupelets of mature fruit. Lesions caused by aecia and uredinia on the bases of primocanes did not become cankerous in the following fruiting year. Necroses associated with large aecia produced a shot‐hole effect and leaflets heavily affected by either aecia, uredinia or telia abscissed prematurely. The sequence of spore stages on pot‐grown plants inoculated with mature overwintered teliospores took 60 days to the reappearance of telia. The rate of development of uredinia and associated necroses was faster on younger than on older leaves but the cumulative spore yield was not significantly different. Optimum temperature for germination of urediniospores in vitro (18.4–20.9°C) was higher than for mature teliospores (14.6–18.4°C). Light inhibited germination of both types of spores. Teliospore germination was inhibited by wavelengths of 590–750 nm, but this was partially reversible under other light conditions. A reversible inhibition of basidiospore formation was also observed under shorter wavelengths.