Tree fruit have been the subjects of genetic improvement for thousands of years. From simply planting seed of the most desirable fruit, to vegetatively propagating the best trees by grafting, to cross-pollination, to the use of gene transfer and genetic mapping, humans have continuously striven for the "perfect" fruit. We have come a long way in our expectations of fruit quality and availability. From times of limited availability, and marketing and consumption of locally produced fruit, we are now in an age where high-quality, blemish-free fruit are expected year-round by most consumers in industrialized countries. Fruit may be grown thousands of miles from the point of consumption, travel weeks to market, and be stored for nearly a year. Breeders strive to develop cultivars that meet the stringent demands ofgrowers, packers, shippers, wholesalers, retailers, and consumers.
Modern fruit-breeding objectives can be divided into two broad classes, those specifically aimed at improving the fruit, and those aimed at improving the tree. Fruit traits include size, flavor, texture, color, disease resistance, and the ability to maintain quality during and following storage. Tree traits include precocity, vigor, size, and resistance to diseases, insects, cold, heat, drought, and flooding. Although tree traits are vitally important for production efficiency, their improvement can be meaningful only if fruit quality is equal or superior to existing commercial cultivars. Therefore, tree fruit breeding has always been concerned with the selection and optimization of complex, multiple traits.
Tree fruit present both unique difficulties and unique opportunities in terms of genetic improvement when compared with herbaceous crops. Long generation cycles and high levels of genetic heterozygosity make the development of improved fruit cultivars a time-
consuming process. For many fruit crops, new cultivars, especially those carrying novel traits, cannot be developed within the span of a breeder's career. For example, peach, a crop with a relatively short generation time of three to four years, has generally required from 10 to 20 years from first fruiting to cultivar release.
The relatively large land areas necessary to grow segregating populations of tree crops add considerable expense to breeding programs. High costs can limit the number of seedlings that are grown, reducing the probabilities of encountering the rare combination of genes necessary to produce a superior cultivar. Once a new cultivar is released to the market it must compete with existing cultivars. The production of some of the most economically important tree fruit relies on the use of a few cultivars. For example, 'Bartlett' accounts for approximately 50 percent of the commercial pear production in North America. Together, 'Bartlett', 'Beurre Bosc', and 'Anjou' account for almost all of the commercial production in the United States. Over 50 percent of the world apple crop is based on 'Delicious', 'Golden Delicious', 'Granny Smith', 'Gala', and 'Fuji'. Sour cherry production in the United States is based almost entirely on 'Montmorency'. These major fruit cultivars are broadly adapted, and a significant body of information exists concerning their production, storage, and marketing. New cultivars, regardless of their apparent superior qualities, lack both an information base and a record of consumer acceptance, making their introduction and adaptation by the industry difficult and slow.
Notwithstanding the difficulties in developing new, successful fruit cultivars, certain characteristics of these species aid cultivar development. Once a desirable phenotype is selected by the breeder, it can be reproduced indefinitely through vegetative propagation. No further breeding is necessary to fix traits in a population as would generally be necessary for seed-propagated herbaceous crops. Vegetative propagation may be through the rooting of cuttings, but more often it is through graftage of a scion cultivar onto a rootstock. This separation of a plant into rootstock and scion genotypes provides a flexibility whereby the scion is not selected for root characteristics (adaptation to specific soils, resistance to soilborne diseases, insects, nematodes, etc.), nor is the rootstock selected for fruiting characteristics. In this way, the selection criteria for any one cultivar of rootstock or scion can be simplified with rootstocks and scions
"mixed and matched" to optimize production and quality over a range of environmental conditions.
Although characteristics under selection for rootstock or scion breeding may differ, the breeding approaches employed are the same. The general approaches to fruit breeding currently in use are hybridization, gene transfer, and molecular marker-assisted selection.
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