Resistance to Phytophthora rubi in Wild North American Red Raspberry Germplasm

Phytophthora root rot is a serious problem in red raspberry production around the world and improving resistance is a major objective in many raspberry breeding programs. The native North American red raspberry, Rubus idaeus strigosus, has been identified as a source of resistance, but is only represented by a few founding clones in modern breeding lines. Little information exists regarding the frequency or distribution of root rot resistance among wild populations. We screened seedlings from 64 wild red raspberry populations collected from across North America for reaction to P. rubi using a semi-hydroponic system in the greenhouse. Root and shoot symptoms were rated on a 1-8 and 1-3 scale, respectively and compared to named varieties known to be resistant and susceptible. Resistance to P. rubi was widespread but varied geographically. Populations collected from eastern North America generally showed few symptoms and had high proportions of resistant plants, while those from western North America were more variable with some showing excellent resistance, some showing apparent segregation for resistance, and others appearing uniformly very susceptible. While further sampling would help to fill in gaps across the distribution of this species, this work indicates that resistance to Phytophthora root rot in wild red raspberry is widespread but variable across North America. These sources of root rot resistance may be valuable in broadening the genetic base of this trait in breeding programs.