Further studies on the occurrence and inheritance of resistance in red raspberry to a resistance-breaking strain of raspberry bushy dwarf virus

A strain of raspberry bushy dwarf virus (RBDV-RB), discovered in England in 1981, readily infects by grafting many raspberry cvs that have gene Bu, which confers strong resistance or immunity to isolates of the common strain. Haida is one of two cultivars that are highly resistant or immune from RBDV-RB, but both its parents, cvs Creston and Malling Promise, are infectible. Studies of the segregation of resistance to both RBDV-RB and a common strain of RBDV (D200) in four progenies related to cv. Haida or its two parents, showed that resistance to RBDV-RB was heritable and occurred when gene Bu was present with a second resistance component whose inheritance is probably multigenic. There was some indication that the second component might be a form of partial resistance to graft inoculation of varying expression, and that cv. Haida possesses this resistance at a high level that has not been distinguished from immunity in the graft inoculations used. Cultivars Creston and Malling Promise possibly have this resistance to a lesser degree, while resistance in cv. Heritage has been distinguished from immunity only by extensive graft tests. Some possible implications for breeding RBDV-RB resistant cultivars are discussed.