554 S. KEDHARNATH 



blister rust resistance in P. strobus L., P. monticola, P. pence Griseb, 

 P. parvi flora Sieb. d, Zucc. and P. walliohiana A.B. Jacks, (syn. P. 

 griffithii McClell.) is largely polygenic since resistance of the inter- 

 species crosses was often expressed in an additive manner. He also reported 

 on a possible major gene controlling rust resistance in P. walliohiana 

 which is expressed as a sloughing off of infected needles. Bingham, 

 Squillace, and Patton (1956) found that rust resistance of some of the 

 resistant selections of P. monticola can be transmitted to the progenies. 

 Bingham, Squillace, and Wright (1960) estimated the broad-sense and narrow- 

 sense heritability of rust resistance in P. monticola as 86 and 68 percent 

 respectively. Bingham (1964, 1966) speculated that blister rust resistance 

 in P. monticola and P. strobus is controlled by polygenes. 



As has been pointed out in the earlier sections of this symposium, 

 it would seem necessary to look for field resistance in the pine species. 

 The aim of mutagenic treatment in this context would be to increase the 

 variability and by subsequent conventional breeding procedures accumulate 

 more and more minor genes for resistance. In this connection the pro- 

 cedures suggested by Borlaug (1966) , regarding different forms of recurrent 

 selection developed by corn workers, could be used with appropriate modi- 

 fications . 



Indeed it may be worthwhile to begin with the selected resistant 

 trees as the basic material. Pollen collected from these trees can be 

 irradiated and used in controlled pollinations of other resistant trees. 

 The resulting progenies could be screened for resistance and the desirable 

 seedlings cloned. These clones can then be used for laying out seed 

 orchards. From these seed orchards we can hope to get further concentra- 

 tion of genes for resistance from which a second wave of more resistant 

 seedlings can be raised. Repeated irradiation in every generation can 

 help in increasing the variability and thus raise the ceiling of resis- 

 tance that can be obtained. 



Interspecific hybrids between the desirable species like P. monticola 

 and P. strobus or between one of these species and other donor species 

 such as P. wallichiana are currently being exploited for increasing resis- 

 tance to blister rust. Little is known as yet about the nature of 

 chromosome pairing in the Fi hybrids. If there is any lack of homology, 

 even if only at the cryptic level, subjecting ¥\ hybrids at the appropriate 

 stage to irradiation may increase the frequency of recombination and thus 

 facilitate the task of transferring genes for resistance from the donor 

 genome to the recipient genome. 



Recent work by Briggs, Flor, Favret, and others has shown that there 

 are numerous genes that often are not distributed at random over the genome 

 of the host plant but tend to be grouped in genetical segments concentrated 

 in a few chromosomes (Gustafsson and Mergen, 1964). If this is the 

 situation in pines, high blister rust resistance may be achieved very 

 quickly if the right block of genes is transferred through interspecies 

 hybridization. Thus, procedures that increase the frequency of recombina- 

 tion may substantially aid the breeder. 



In conclusion it may be said that artificial mutagenesis cannot replace 

 hybridization and recombination. It is an additional tool. However, its 

 significance will increase to the extent we learn to predict the chromo- 

 some and gene changes which are produced by various mutagens (Gustafsson, 

 1960) . 



