BACKCROSS BREEDING WALNUTS FOR RESISTANCE TO THE CHERRY LEAFROLL VIRUS
In 1983, a backcross breeding program to transfer hypersensitivity from the rootstock species, J. hindsii to J. regia was initiated. The goal is to develop CLRV resistant cultivars with English walnut quality. The initial BC1 population consisted of 82 Paradox x English walnut [(J. hindsii x J. regia) x J. regia] seedlings. To determine the inheritance of resistance 10 Paradox and 16 Paradox X J. hindsii, and J. regia and J. hindsii controls were included.
Hypersensitivity from black walnuts segregated as a single dominant gene. Bark patch testing of 82 BC1 trees resulted in 42 individuals tolerant of CLRV and 40 which were hypersensitive to the virus. This conforms to the expected 1:1 segregation. All Paradox, Paradox X J. hindsii and J. hindsii were hypersensitive, whereas J. regia was systemically infectible (tolerant).
Backcrosses to the hypersensitive BC1 trees using the English cultivar Chandler as a pollen parent produced over 200 BC2 trees. Of those tested to date, 77 are tolerant and 79 are hypersensitive. The most limiting trait is shell thickness.
Parents of BC3s have been selected on the basis of hypersensitivity, precocity, and nut and shell quality. Eight mature BC2s, which were confirmed to be hypersensitive by bark patching, have been used as parents to produce 101 BC3 seedlings.