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ACCELERATING ORCHARD EVALUATION OF APPLE ROOTSTOCK CANDIDATES
Typically 25 to 35 years have been required between the production of seedling progenies from controlled crosses and the eventual introduction of a new cultivar or new rootstock selected from those seedlings.
The orchardist needs more rapid responses to his needs than the breeder has been able to provide, yet premature introduction has almost invariably resulted in economic loss to the orchardist, rejection of the new clone, and substantial erosion of the credibility of both breeder and his institution.
Field evaluation as practiced by many breeders involves three stages: 1) original seedlings are set very closely in the row and allowed to fruit; 2) selections are grown as grafted trees at normal orchard spacing at one location, usually the breeder’s station; and 3) as a "second test", elite selections are grown under commercial conditions at a number of locations (Layne and Quamme, 1975). Cooperation among members of groups such as the Apple Breeders Cooperative leads to exchange of scionwood of new candidates very early in the selection process and facilitates evaluation at numerous locations (Hough, 1979). The pear breeding team at Harrow, Ontario, now bypasses the traditional one-location "first test" and moves initial selections into experimental plantings at a number of locations at a very early stage (Layne and Quamme, 1975).
Three categories of evaluation can be identified which cause the extended time between production of hybrid seed and introduction of clonal rootstocks: 1) screening for resistance to biotic and physical hazards of the environment, with major emphases given to factors thought to be limiting; 2) evaluation for adapatability to soils and soil conditions; and 3) evaluation for horticultural performance, both in the nursery and in the orchard.
In breeding tree crops, very large progenies are essential if adequate opportunity for the necessary genetic recombinations is to be afforded (Brown, 1975; Cummins and Aldwinckle, 1974, 1979; Knight, 1962). Early screening for susceptibility to hazards can be used to reduce large populations to tolerable levels.
Nursery evaluation for propagability, freedom from spines and burrknots, and thriftiness requires about 5 years.
The objectives and prescreening techniques of