610 SCOTT S. PAULEY AND CLIFFORD E. AHLGREN 



INTRODUCTION AND SUMMARY 



The breeding project here briefly described has been designed pri- 

 marily for a low budget. The design also makes it feasible to utilize a 

 broad genetic base of selections in the first phase of the program. 



The initial phase consists of the phenotypic (mass) selection of 

 about 1,000 Pinus strobus L. mother-trees in the high rust infection 

 areas of northeastern Minnesota. About 100 plants grown from open- 

 pollinated seed of each of these selections (i.e., ca. 100,000 plants) 

 are being propagated and outplanted on a high infection site near Tofte, 

 Minnesota, and subjected to a natural screening for rust resistance. 



The surviving plants will be allowed to inter-pollinate and the seed 

 may be used for commercial outplantings . Genetic gain in resistance may 

 be expected to be rather low at this stage, possibly in the order of 

 12 to 15 percent. 



Flowering by the age of 15 to 20 years in the surviving plants of 

 the screen test will permit initiation of the final recurrent family 

 selection phases of the improvement program. Total genetic gains in such 

 full-sib family selection may hopefully be expected to range from 30 to 

 40 percent in the first generation and possibly higher in succeeding 

 generations . 



In spite of a necessarily lengthened time schedule the advantages of 

 the proposed initial mass selection-screen test phase may be summarized 

 as follows: 



1. The genetic base would be much broader because of the large num- 

 ber of trees initially screened. This involves improvement considerations 

 other than rust resistance since, in the family selection phases of the 

 program, traits such as growth rate, branching characteristics, weevil 

 resistance, etc., may logically be incorporated. With such a broad 

 genetic base, isolation of rust-resistant lines combined with other 

 desirable traits would be easier than in a population initially restricted 

 in size. 



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2. Due to uniformity of treatment in the screen test the possibility 

 of highly susceptible "escapes" would probably be much lower than in wild j 

 trees growing in widely diverse habitats. 



3. The fact that the surviving trees in the screen test will be of 

 the same age, uniform in size when sexual maturity is reached, and close 

 together will greatly reduce controlled pollination costs, especially 

 labor for bagging, travel, etc. 



4. Uniform age of the stock and design of the screen test (randomized 

 complete blocks) will permit tests of fertilizer and other treatments to 

 hasten flowering of surviving plants. 



