SELECTION PHILOSOPHY AND METHODS 



New varieties of western white pine resistant to blister rust must include as 

 many of the known kinds of resistance as possible (table 4). Natural host :parasite 

 systems evolve an optimum balance of the various mechanisms of resistance (Harlan 

 1976). We hope to anticipate such a balance in the proposed selection scheme, or at 

 least come close enough so that natural adjustments could be accomplished without 

 serious disruptions. 



We have drawn freely from the literature and experience of the crop breeders to 

 develop a selection scheme. Although there may be major differences between forest 

 tree species and crop plants, in principles of selection they are very similar. The 

 following principles exemplify the prevailing philosophy of the interaction host -.parasite 

 systems: 



. 1. Resistance is the rule, susceptiblity is the exception. 



2. Mechanisms of resistance vary in type and frequency. 



3. Some mechanisms of resistance bestow to their carriers complete resistance, 

 others slow the disease or curtail its development. 



a. Mechanisms that bestow complete resistance are typically controlled by 

 single genes and are called vertical or differential resistance. 



b. Mechanisms that slow or curtail disease are typically polygenetical ly 

 inherited and are called horizontal, uniform, or field resistance. 



4. Vertical resistance can be neutralized by a complementary gene in the 

 disease-causing organisms. 



5. Horizontal mechanisms of resistance are not neutralized by new races; 

 however, more aggresive races might decrease the effectiveness of the horizontal 

 resistance, 



6. Combinations of several vertical resistance genes exhibit characteristics 

 of horizontal resistance. - . ' 



7. Disease-causing organism are also restricted by their own natural fitness 

 requirements. 



8. Resistance is the observed expression of the interaction of host genes, 

 pest genes, and environment. The level of resistance required changes with environ- 

 ment,, weather, and disease cycle. Low levels of genetic resistance in a population 

 often result in high levels of phenotypic expression. 



9. Resistance composed of horizontal types exhibits more stability than 

 resistance composed of vertical types. 



These principals lead to the following guidelines for development of new varieties 

 of western white pine: 



1. Never base resistance on only one gene. A variety of western white pine 

 could be produced that would be 100 percent resistant, but it probably would not last 

 long. 



2. Use combinations of resistance types--diversity is essential. 



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