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Yearbook^ of Agriculture 1949 



ing trees resistant to particular fungus 

 and virus diseases. The development 

 of new forms resistant to insect ene- 

 mies, however, has scarcely made a 

 beginning. 



The need for the work is clear 

 enough. Besides the losses we have in- 

 curred, in some regions of the United 

 States forest planting is coming into 

 use as the surest and quickest method 

 of reproducing the desired wood crops. 

 Planting makes it possible to control 

 the kind and variety of tree that oc- 

 cupies the site and gives special em- 

 phasis to the need for careful selection 

 of the planting stock. It costs no more 

 to plant the resistant trees, if they are 

 available, than to plant ordinary stock. 



In the development of trees resist- 

 ant to a particular disease or insect 

 enemy, the same principles of selection 

 and genetics apply that are employed 

 in the development of new, vigorous, 

 and fast-growing tree forms. The tree 

 breeder, however, usually desires both 

 resistance and vigor in his final selec- 

 tion, but when the laws of heredity de- 

 cree differently, the breeder faces a 

 difficult problem. The solution of such 

 problems requires knowledge of several 

 sciences, especially genetics, plant path- 

 ology, entomology, and forestry. 



The breeder first attempts to select 

 trees that show resistance to the partic- 

 ular pest under study. Resistant selec- 

 tions are propagated by grafting or by 

 cuttings. Seed from such selections is 

 collected and thousands of seedlings 

 are grown in nurseries where they may 

 be tested against the pest, or the seed- 

 lings may be transplanted to testing 

 plots where they can be tested at a 

 more suitable age. 



Sometimes selection results indicate 

 that no individuals of the particular 

 species or of related species are re- 

 sistant. It is then necessary to import 

 seed of foreign species for testing. The 

 related foreign species, however, may 

 possess no worthy characteristic other 

 than that of resistance. The breeder 

 must combine this character of re- 

 sistance with the desirable characters 

 of the susceptible species. The first step 



to bring about this combination is to 

 produce a hybrid by crossing a resistant 

 tree with a susceptible tree. 



Hybrids obtained from the first 

 crossing of two varieties or species are 

 known as first filial (Fi) generation 

 hybrids. If an F tree sets seed by its 

 own pollen (selfing) , or if two or more 

 FI trees are crossed with one another 

 (sib-mating) , the resulting hybrids be- 

 long to the second (F 2 ) generation. 

 The F 2 and subsequent generations are 

 called the segregating generations be- 

 cause all the characters, visible and in- 

 visible, that were present in the Ft 

 trees segregate out among the various 

 trees of the later generations. 



RESISTANCE to a pest may be in- 

 herited in one of three ways. If resist- 

 ance is inherited as a dominant char- 

 acter, all the FI trees will be dominantly 

 resistant and most of the F 2 trees will 

 be similarly resistant. Resistance may 

 be inherited as an incomplete domi- 

 nant, in which instance the FI trees 

 will be more or less intermediate in 

 their resistance to the pest. The Fj. 

 trees as a group will not show the re- 

 sistance of the resistant parent nor the 

 susceptibility of the other parent. In 

 this type of inheritance the second and 

 subsequent generations will produce a 

 lower proportion of resistant trees than 

 the first type of inheritance produces. 

 If the breeder meets either one of these 

 types of inheritance, he will have com- 

 paratively little difficulty in obtaining 

 trees with a satisfactory degree of re- 

 sistance. But susceptibility may be in- 

 herited as a dominant character. The 

 first-generation trees will be susceptible 

 and will have no value except for fur- 

 ther breeding to obtain second-genera- 

 tion trees. The second generation in 

 this instance must consist of large num- 

 bers of trees because the proportion of 

 resistant specimens will be exceedingly 

 small. 



In agricultural crop breeding, the 

 breeder usually fixes the type by in- 

 breeding so that it reproduces more or 

 less true from seed. The tree breeder 

 cannot afford to fix his hybrid types. 



