584 ERNST J. SCHREINER 



The average genetic improvement of all the progenies (full-sib or 

 half-sib) represented in the mass-produced planting stock derived by seed 

 from improved varieties — single- or multiple-cross hybrids, multilineals , 

 or synthetics--will determine the genetic gain. Therefore, genetic 

 improvement of such varieties will require parental populations with high 

 general combining ability, not only for special traits but also for a high 

 degree of fitness. How long will it take to create a "practically" true- 

 breeding variety of Pinus strobus that combines a reasonably high degree 

 of fitness with blister rust resistance, white pine weevil resistance, 

 rapid growth, and good timber quality? 50 years? 100 years? And how 

 long could it be maintained without alteration? 



Exceptionally superior individuals may be obtained by various 

 intensive breeding methods, or by extensive selection in progenies derived 

 from gene pools or natural populations. Patton and Riker (1966) found 

 that rust-free Pinus strobus ortets selected in natural stands showed a 

 broad range of susceptibility in clonal tests, from high resistance (if 

 not immunity) to extreme susceptibility. 



Multiclonal varieties will depend upon the selection of individuals 

 for a high degree of inherent fitness and desired inherent attributes, 

 not on the average inherent performance of families or lines. When 

 individual clones in a multiclonal hybrid variety begin to lose their 

 value, due to increasing disease or insect susceptibility, lack of local 

 adaptability, decline of general fitness resulting from long-term 

 environmental changes, or to change in industrial use requirements, they 

 could be replaced by new clones selected from the appropriate regional 

 gene pool. Such clonal changes could be made on short notice because the 

 breeder could multiply superior genotypes for commercial use without 

 adulteration of the genotype, and without determining their combining 

 ability to transmit the desirable qualities or characteristics. Seed 

 orchards and progeny test plantations will provide unusually extensive 

 and diverse gene pools for the selection of ortets for clonal testing 

 for use in multiclonal hybrid varieties. 



The breeding procedure to obtain multiclonal hybrid varieties should 

 follow the same general pattern of selective intraspecific breeding, 

 including sib- and backcrosses, species hybridization, and recurrent 

 selection as for the development of synthetic varieties to be propagated 

 by seed; but there should be a difference in the nursery procedure and 

 the progeny tests. As large a number of selected seedlings as possible 

 (Fi and later generations) should be released from competition and grown 

 in the nursery beds or, preferably, in a transplant nursery long enough 

 to produce a sufficient number of ramets for replication of at least 

 2-tree clonal plots in several regional progeny tests. This would 

 increase the size of the progeny test plantations, but they would then 

 also be clonal tests, and by proper roguing, gene pools of superior 

 genotypes. The clonal tests would be the source of superior clones for 

 use in multiclonal hybrid varieties and for continuing individual- and 

 mass-pedigree breeding. 



Depending on the growth rate and reproductive habit of the species, 

 the inherent characteristics or qualities to be improved, and assuming 

 the availability of a sufficient number of parent trees, the total time 

 required to complete one breeding cycle in the production of synthetic 

 seed varieties, and to produce first-cycle ortets for mass propagation 

 in multiclonal hybrid varieties might be approximately as follows 

 (Schreiner, 1966a, b) : 



