BREEDING FOR RESISTANCE TO THE BLAcnINE VIllUS: INCREASING THE SPEED AND EFFICIENCY OF TRADITIONAL BACKCROSSINGBY INTEGRATING KOLECULAllTECHNIQUES

Walnut blackline disease caused by the cherry leafroll virus (CLRV) causes fatal necrosis at the union between English walnut scions and hypersensitive rootstocks. All English walnut cultivars tested so far are susceptible to blackline girdling. A breeding program for developing a blackline resistant Englishwalnut cultivarwas begun in 1983. The time requiredto breed an English walnut cultivar that is resistant to CLRV can be significantly decreased by integrating new moleculartechniqueswith traditionalbackcrossing. The current blackline resistance breeding stock is a population of about 140 BCl and BC2 items and their parents. To increase this population, over 900 controlled crosses were made in 1991. The resulting seeds (307 BC2 seeds and 4 BC3 seeds) will be planted to a nursery in the spring of 1992. Selection among these seedlings based on seedling morphology and phenology, as well as on seedling DNA, will permit muchmore rapidprogressthansimplebackcrossing. Morphological and phenological data was taken for 24 different traits on all the items in the blackline breeding program, as well as for hybrid seedlings. Crackou,t, yield and nut quality data were taken for all mature items in the program. The observed variation was correlated with the percent English walnut genome present in the item. DNA was collected and isolated from about 75 members of the breeding population. Analysis of the DNA has uncovered 10 DNA polymorphisms that can be used to determine which backcrossseedlings aremostpromising. 'rwenty-seven BC2 accessions were grafted and inoculated to determine if they are resistant to CLRV.