PATHOLOGY AND GENETICS OF WHITE PINE BLISTER RUST 441 



indicated that structurally it was not a very efficient organ for the 

 reception of external pycniospores and their contact with internal 

 flexuous hyphae . Also, under his conditions, aeciospores were produced 

 without the exchange of pycniospores between pycnia on different cankers. 

 He concluded that if heterothallism in C. ribicola requires the exchange 

 of compatible pycniospores between pycnia prior to the formation of 

 aeciospores, his results did not indicate the fungus to be heterothallic. 

 Conclusive proof remains to be obtained by making pycnial exchanges 

 between isolated cankers known to be of single-basidiospore origin, or 

 by other approaches as noted by Bingham (1969) . 



RACES OF C. RIBICOLA 



New races of such organisms as our tree rusts are probably being 

 formed continually through mutations in pathogenicity and recombinations 

 through sexuality, parasexuality, and heterokaryosis . The survival and 

 increase of a given race primarly depend on the resistance of a host. A 

 change in the host substrate, such as the sudden production of resistant 

 trees, may lead to conditions favorable for a race that previously could 

 not have survived or become important. We know that various biotypes of 

 Cronartiton species now exist, but we have no information on the likeli- 

 hood or the speed with which a race could shift in prevalence. 



There is some evidence that physiologic races of C. vibioota do 

 exist. Hahn (1949) was unable to find evidence of pathogenic races on 

 immune Ribes in Canada. Anderson and French (1955) , however, reported 

 differences in virulence as evidenced by a necrotic reaction on a clonal 

 line of Ribes hivtetlvm Michx. between aeciospores collected from sugar pine 

 in the West and those from eastern white pine sources in the East and 

 Midwest. Differences in morphology of germ tubes of aeciospores from 

 three different collections in neighboring white pine stands were con- 

 sidered as evidence of physiologic races by Straib (1953) . Recently, 

 Bingham (1969) reported finding different colored needle lesions resulting 

 from C. yyibicola infections on the same western white pine needle and 

 plant. All of these findings point to variation in C. ribioola, but 

 there is no knowledge yet of variation in pathogenicity on pine, the 

 feature that is of most importance to our efforts in resistance breeding. 

 Determining the existence of pathogenic races on pine involves diffi- 

 culties in developing adequate methods for their detection. But as our 

 understanding of the pathology and genetics of both the host and parasite 

 increases, it is likely that we can obtain a more precise estimate than 

 we now have of the possible impact of pathogenic races on the development 

 of resistant white pine stock. 



SUMMARY 



The development of blister rust-resistant white pines depends to a 

 large measure upon our understanding of both internal and external 

 influences on the behavior of C. vibicola, as well as a knowledge of 

 resistance in the host. 



Infection is tied in closely with conditions influencing basidiospore 

 production and germination. The production of basidiospores depends not 

 only on conditions for teliospore germination but also on those during 

 teliospore formation. Since infection does not always follow spore 

 germination, further clarification is needed of the significance of 



