Based on hardiness of wood to ultimate cold, the tree fruits are ranked as follows from 

 the most to the least hardy: American plums, apples, pears, sour cherries, sweet cherries, 

 apricots, European plums, peaches, and Japanese plums. Sweet cherries are susceptible to 

 early freezes occurring before they can harden. And although both the wood and fruit buds 

 of apricots will withstand more cold than peaches when they are dormant, apricot blossoms 

 are susceptible to freezes during spring bloom. 



Besides the variations in cold hardiness between the different kinds of fruits, there is con- 

 siderable difference in cold hardiness among varieties within a particular kind of fruit. Plant 

 breeders and explorers seek to capitalize on this difference to extend areas in which the vari- 

 ous fruits can be successfully grown and to cut down on freeze damage through the discovery, 

 development, and introduction of hardier fruits having other desirable characteristics. 



As in the case of citrus, commercial varieties of many deciduous fruits are grown on 

 rootstock selected for hardiness, disease resistance, vigor, dwarfing effect, or other traits con- 

 tributing to economic production. However, some rootstocks that are desired because of other 

 characteristics are susceptible to cold damage. When trunks of the desired rootstock variety 

 are subject to cold damage, a hardy interstock is sometimes used between the rootstock and 

 the scion. Such a double-grafting technique is a more time-consuming and cosdy process 

 than grafting directly on rootstock. Consequently plant breeders are working to incorporate 

 hardiness in some non-hardy but otherwise desirable rootstocks. A case in point is a root- 

 stock that is badly needed because of its resistance to destructive pests but is susceptible to 

 cold. 



A major obstacle in the search for hardy deciduous tree fruits having other desired hor- 

 ticultural traits is the relatively long time required to bring promising new selections into pro- 

 duction and to evaluate them. In some cases fruit growers have only recently begun to reap 

 the benefits of plant exploration, breeding, and testing experiments begun many years ago. 



Status of cold hardiness research on some of the more common deciduous tree fruits 

 grown in various sections of the United States is reported briefly in this section. 



Apples 



Although apples rate ahead of most other deciduous tree fruits in ability to withstand 

 cold damage, increased winter hardiness has long been sought in selecting and breeding varie- 

 ties for most of the apple belt and is an all-important requirement for successful apple pro- 

 duction in colder sections such as the northern Northeast and the Northern Great Plains. 



The selection and breeding of hardier apples have been conducted on a systematic basis 

 in this country for more than half a century, with plant scientists of USDA, interested States, 

 and industry participating. At the present time the bulk of the Department's search for har- 

 dier apples having other desirable traits is limited to work at the Northern Great Plains Field 

 Station, Mandan, N. Dak. 



As many as 30 to 40 years are required to get a new apple variety from seed into com- 

 mercial production. However, hybridization experiments begun many years ago are beginning 

 to pay off in a number of varieties combining cold hardiness with other desired characteristics. 

 In 1959, the Mandan field station named and introduced four reliably hardy apple varieties 

 adapted to growing in most sections of the Northern Great Plains. Other selections devel- 

 oped in the breeding program are in advanced stages of testing and expected to merit introduc- 

 tion. All four hardy varieties introduced by the Department at Mandan were hybrids result- 

 ing from crosses using one or two hardy parents. The new varieties and their parentage are: 

 Garrison and Thorberg, both made by crossing Duchess and Starking Delicious; Peach Gar- 

 den, a cross of Melinda and Duchess; and Killand, a hybrid of Mcintosh and Dolgo. 



Another approach to the problem of developing improved apples is by the use of mutant 

 strains. One objective in the selection, propagation, and testing of bud mutations has been to 

 increase the ability of already established varieties to survive and produce in northern areas. 



