Corresponding author: K

The fungal pathogen Cronartium ribicola J.C. Fischer, causal agent of white pine blister rust (WPBR), was introduced to the eastern United States about 1898 (11). Hosts for the fungus include fiveneedle pines (Pinus sp. section Quinquefoliae) and currants and gooseberries (Ribes sp.). Natural infection of wild populations of Pedicularis bracteosa Benth and Castilleja Mutis ex L.f. have also been documented, although these two genera are considered minor hosts of the disease (14,20). The damaging effects of WPBR on host white pines diminished the appeal of Ribes cultivation in the United States for most of the twentieth century; however, Ribes crops have been a mainstay of the small fruit industry in Europe for decades (1). The fungus produces pycnia and aecia on five-needle pines, and uredinia, telia, and basidia on Ribes (13,15). Aeciospores, produced from cankers on diseased pines, and urediniospores, originating from uredinia on abaxial leaf surfaces of susceptible Ribes, can infect Ribes foliage. Susceptible Ribes may be defoliated by C. ribicola where infection incidence is high (3). Pine hosts can be killed by WPBR stem canker infections. Resistance to WPBR has been identified through the action of dominant genetic inheritance in aecial hosts (11,12) and telial hosts (1,2,4–8,16). Although Richardson et al. (16) have reported infection in Ribes nigrum L. by both spore types, they did not report the response of different spore types at the Ribes clonal level. Differential infectivity of these spore types could change breeding strategies for developing new Ribes cultivars resistant to WPBR. Previous horticultural studies have examined Ribes susceptibility to urediniospores (7–9). An unanswered question is: do aeciospores and urediniospores infect Ribes genotypes equally? The objective of this study was to determine whether both spore types were infective to a set of F1 genotypes of R. nigrum with known diverse WPBR response.