532 RAYMOND J. HOFF AND GERAL I. MCDONALD 



CORRELATION OF FACTORS FOR RESISTANCE 



Information on factors for resistance is still scanty but striking 

 similarities occur within the pines in response to infection by the 

 various rusts. Fewer lesions per unit area of infection court and slower 

 fungus growth appear to be the most common factors so far observed. The 

 needle-spots-only traits, either in secondary needles or primary leaves, 

 also appear frequently. Prevention of penetration of secondary needles 

 appears to occur throughout the hard pines and at times within the soft 

 pines. Canker death has been reported for many of the soft and hard 

 pines . 



Admittedly this information is fragmentary. Even so, we are led to 

 ask, "Have similar resistance factors evolved in the species of a genus 

 that ward off diseases caused by related pathogens?" In other words, is 

 there one set of factors for resistance that protect the pines from the 

 stem rusts? While there may be one set of factors, different combinations 

 may be found within the various host-parasite systems. However, exactly 

 the same combination of factors for resistance may have evolved to 

 protect closely related host species against a particular rust. If this 

 is so, knowledge concerning one part of the total pine-rust system would 

 be very useful to another part. For example, similar factors would be 

 hypothesized and thus more easily found and studied. A breeding scheme 

 based upon this principle may enable the tree breeder to establish more 

 easily a balance in an unbalanced system or prevent an imbalance in a 

 presently balanced system. 



PROPOSED USE OF NATURAL MODELS 



We propose that a new resistant population of western white pine be 

 patterned after the combinations of factors that are presently protecting 

 the highly resistant white pines (P. armandii 3 P. w alii chi ana 3 P. koraiensis, 

 and P. cembra) from severe blister rust damage. This scheme would take 

 advantage of the fact that the factors for resistance in the highly 

 resistant white pines have already stood the test of time and are present 

 in pines very closely related to western white pine. Some or all of 

 these factors may be useful. However, there may be other factors in 

 western white pine that may be more useful. Eventually all factois should 

 be tested for stability. But the beauty of using "balanced symbiosis" as 

 a model is that it allows the tree breeder to follow a priority list. He 

 can select first those factors that are stable under "balanced symbiosis," 

 and in the meantime be testing the stability of new factors. 



It is important to point out that selection of a factor for resis- 

 tance, by using the model system, would be based on its ability to 

 protect western white pine from blister rust while remaining stable and 

 not on the number or kinds of genes involved. 



Of course., there is a great deal of information needed before we can 

 begin to utilize the model. This would be equally true for other breeding 

 methods. But in using "balanced symbiosis" we are following a natural 

 plan that requires less speculation than the use of more artificial plans. 

 Also, studies of "balanced symbiosis" could yield information of great 

 value in understanding the evolution of host-parasite systems. 



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