526 RAYMOND J. HOFF AND GERAL I. MCDONALD 



population containing adequate levels of long-lasting resistance. Thus, 

 the tree breeder's problem lies in gaining enough understanding of resis- 

 tance so that a correct decision can be made before years of investment 

 have accumulated on a product destined for probable failure. 



The purposes of this paper are: (1) to summarize the factors for 

 resistance observed in blister rust-resistant western white pine (P. 

 monticola) ; (2) to discuss recently proposed agronomic schemes for 

 incorporating factors for resistance into a long-lasting product; (3) 

 to present an approach to the problem of breeding blister rust-resistant 

 western white pine based on the natural models of resistance available 

 in many tree-rust systems; and (4) to outline the specific knowledge 

 required to implement a breeding program aimed at producing blister rust- 

 resistant western white pine. 



PHILOSOPHY OF DISEASE RESISTANCE BREEDING 



In recent years decided changes have occurred in the philosophy of 

 breeding for disease resistance in plants (van der Plank, 1963, 1968, 

 1969; Zadoks 1 ). These changes place emphasis upon long-lasting but 

 undramatic uniform (also termed general, horizontal, and field) resis- 

 tance and tolerance instead of the more dramatic differential or specific 

 resistance. This change of emphasis came about because of the general, if 

 not universal, breakdown of differential or specific resistance by viru- 

 lent races of the pathogen. 



Lessons learned by the breeders of agronomic crops are important for 

 forest tree breeders. The 120-year rotation used in the past by the 

 Forest Service for western white pine in northern Idaho might be reduced 

 to 60 years on the better sites, but this rotation would still require the 

 use of resistance genes with a life expectancy of at least 30 to 40 years. 

 This is far beyond the average life span of differential resistance genes 

 in annual crop plants. 



Presently, long-lasting uniform resistance, together with tolerance, 

 appears to offer the surest path of success. This should lead to the 

 concomitant survival of all members of the system. In other words, we 

 must learn to live with the pathogen. Our first objective should be to 

 produce new populations of white pines that have at least a degree of 

 resistance necessary to make their culture profitable. 



THE DISEASE CYCLE 



The disease cycle of blister rust has been covered previously 

 in this symposium by Dr. Patton, but its importance to the study of 

 resistance is great enough to justify a summary. The fungus generally 

 enters its pine host through stomata of the primary and secondary leaves, 

 but entry through succulent stem tissues is also possible, at least under 

 controlled conditions. Once in the leaf the fungus usually grows toward 

 the stem via the vascular cylinder. Rapid spread in the living tissues 

 of the stem produces the characteristic fusiform canker. Production of 

 aeciospores takes place within the margins of the canker, and complete 

 girdling of the main stem by the fungus finally results in death of or 

 severe damage to the tree. 



Paper presented in these proceedings . 



