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FLOOR DISCUSSION 



BECKER: In the case of the blister rust resistance work of the 

 Intermountain Station here in Moscow, the rust has been present and 

 selection has been proceeding almost 50 years. I would like to ask 

 Dr. Stern, do you think that the blister rust organism itself will change, 

 and therefore do away with genetic advance they have made? 



STERN: The white pine is part of the environment of blister rust 

 here, and if you change the environment of an organism it will react 

 genetically--but you can't predict how it will react. There are, of 

 course, cases known where the parasite tended to be less violent in its 

 interaction with the host population. But nothing is known about the 

 genetic background of this, and nothing is known as to which feature of 

 specific genetic combination the host and parasite must have to give this 

 result. To answer correctly we would need to know more about the bio- 

 logical facts relevant to this particular disturbed, disequilibrated 

 host-parasite system. 



HATTEMER: I didn't get a clear understanding as to why there must 

 be genetic correlations between any unknown traits that as a by-product 

 bring about resistance— if the phenomenon of recruitment holds. 



STERN: Well, if you make things too clear people will think it's 

 no longer scientific. I will try to give you a clearer example. In 

 birch rust, for instance, the phenology of different birch provenances 

 may be quite distinct. Some provenances are early starters, and normally 

 these cease growth quite early in the fall. Other provenances start late 

 but keep growing until the end of October. If birch rust infection 

 occurs early, say before the middle of September, then the late starters 

 have some healthy leaves. Resistance is lower in the early starters 

 where all leaves are infected. This example of the correlation between 

 cessation of growth and resistance is only one of many. 



