STEM RUSTS OF CONIFERS .AND THE BALANCE OF NATURE 529 



few at a time, until finally all factors for resistance are again 

 neutralized. 



Scheme 2 



This scheme involves the formation of multilineal varieties, hybrids 

 or a large rumber of varieties, each with a separate factor for differ- 

 ential resistance. These are then released in one area where it is known 

 that the complementary rust races are largely absent (Borlaug, 1959, 1965 

 1966) . This scheme requires a constant awareness of the frequency of 

 the genes for virulence in the pathogen populations , and varieties have 

 to be changed periodically before new epidemics arise. 



Scheme 



A third scheme utilizes uniform resistance, together with tolerance 

 reactions, as a base upon which to build differential resistance [Hooker, 

 1967). Under this scheme the breeder has assurance of obtaining a 

 merchantable crop even though certain of the differential types may be 

 broken down. However, van der Plank (1965, 1968) strongly suggests that, 

 because of the "vertifolia" effect (a loss of uniform resistance coupled 

 with selection for genes controlling differential resistance) , selection 

 should be based solely on uniform resistance and/or tolerance reactions. 



Of what value are these schemes in breeding blister rust resistance 

 in western white pine? Scheme 1 could be useful since there are several 

 factors for resistance in western white pine, some of which are controlled 

 by single genes. These genes could be incorporated into a single variety 

 that might withstand infection for many years. This is not likely, how- 

 ever, since the bridges are already present in the natural population 

 and we can assume that these genes would have the same fate as most single 

 differential genes in the crop plants. On the other hand, if some of the 

 new hypothesized races had a low level of fitness on the alternate host 

 {Ribes spp.) the resistance genes complementary to them would have a much 

 greater potential. Moreover, certain factors for resistance may have 

 great stability because of certain features of the blister rust disease 

 cycle in western white pine. That is, if the fungus can enter the stem 

 under natural conditions (at least at low levels of infection) , the fungus 

 would be able to complete its life cycle by circumventing resistance 

 factors in the needle. Together these conditions would mean: (1) selec- 

 tion for virulent races complementary to the factors for resistance 

 located in the leaves would be decreased by allowing the original races 

 (races presumably with higher levels of fitness) to complete their repro- 

 ductive cycle; and (2) the resulting level of stem infection would be low 

 enough to allow production of an adequate timber crop because direct stem 

 infection seems to be rare and would probably tend to be located at the 

 end of branches. This is where the largest amount of succulent stem 

 tissue would be available. 



Scheme 2 holds very little promise. The relatively long rotation of 

 western white pine prevents the continued and rapid change of host geno- 

 types. Nevertheless, this scheme could be used if the new hypothesized 

 virulent races had low fitness on Ribes in comparison to the less complex 

 races. This, coupled with a mixture of many factors for resistance 

 throughout the host populations, could mean a profitable harvest. 



