24 JOHN W. DUFFIELD 



amount of genetic variation in infectivity of the pathogen and in suscepti- 

 bility of the host species. At present, we have rather limited evidence 

 bearing directly on this question, other than the successes which have 

 been achieved in current breeding programs. Of particular interest is the 

 question whether white pine blister rust resistance transferred from 

 Eurasian white pines may offer protection against a wider spectrum of 

 races of the pathogen than resistance recovered from indigenous pine popu- 

 lations. Until we have experimental evidence, speculative answers may be 

 of some value in planning breeding strategy. One line of speculation 

 follows . 



In the early 20th century, Cronartium ribioola J.C. Fisch. ex Rabenh. 

 did not exist in North America, but pines susceptible to this parasite 

 existed in North America and Eurasia. The consensus of evidence and con- 

 jectures on the origins of pines postulates an Arctic origin, with 

 southward migration in North America and Eurasia (Mirov, 1967). Mexico 

 appears to have been a secondary center of active speciation. By the 

 beginning of the historical era, American and Eurasian pines were dis- 

 junct. To account for the relations between white pines and C. ribioola 

 in the early 20th century, there are three logical hypotheses: 



(1) Extinction of C. ribioola in America after it came to America 

 with pines or after it went to Eurasia with pines. 



(2) Migration of pines to America, leaving C. ribioola in Asia. 



(3) Development of C. ribioola in Eurasia after pines migrated 

 to America or after they arrived in Eurasia. 



A fourth, no doubt trivial, hypothesis is that the parasite existed before 

 the host. Leaving this one aside, it is possible to work with the first 

 three. The reactions of American white pines to C. ribioola suggest that 

 host and parasite had met before. Therefore the hypothesis of evolution 

 of C. ribioola after migration of white pines between Eurasia and America 

 seems implausible. Hence it seems likely that there has been a long 

 period, prior to the present century, when C. ribioola had the opportunity 

 and was under evolutionary pressure to develop races to cope with resis- 

 tance mechanisms in its Eurasian host species. The introduction of C. 

 ribioola to America presumably brought a small number- -perhaps only one-- 

 of these races. This race, or these few, have had little time, especially 

 in view of the relatively long cycle of this parasite, to evolve new races, 

 Thus it may be reasonable to expect that Eurasian white pines have a more 

 comprehensive resistance to C. ribioola than the American species, and 

 that resistance transferred to American pines by hybridization may be of 

 longer practical duration than resistance already existing in American 

 pine populations. This supposition is partly testable by growing resis- 

 tant American lines under exposure to the pathogen in Eurasia. 



With reference to Cronartium fusi forme Hedge. $ Hunt ex Cumm. , it is 

 interesting to note that the host species, principally P. taeda and Pinus 

 elliottii Engelm. , appear to be related to the hypothetical Mexican center 

 of pine speciation, having no relatives in Eurasia, just as the alternate 

 hosts, oaks of the sub-genus E ry throb al anus , occur only in America and 

 are also especially numerous in Mexico. 



The characteristics of the breeding materials and problems I have 

 been discussing could be summarized in several ways. I have chosen to 

 summarize them so as to support my strongly held prejudice that breeding 



