IMPROVED PLANTING STOCK THROUGH SYNTHETIC VARIETIES 577 



Southern Pines 



There is sufficient individual resistance to fusiform rust in P. 

 taeda and P. elliottii to justify intraspecific breeding. It is 

 doubtful, however, whether the gain to be expected from intraspecific 

 breeding would be as great as might be expected from species 

 hybridization. 



Pinus eohinata Mill, x P. elliottii and P. eahinata x P. taeda Fj 

 hybrids are highly resistant under natural exposure. The Southern 

 Institute of Forest Genetics and the U.S. National Forests in the 

 Southern Region have a cooperative program to produce the P. eohinata x 

 P. elliottii hybrid in quantity for planting on National Forest areas 

 where rust incidence is high (Henry and Jewell, 1963). 



Although the P. eohinata x P. elliottii hybrid is very promising, 

 Snyder and Squillace (Quoted by Wakely, Wells, and Campbell, 1966) 

 reported that seed yields from controlled pollinations averaged only 8.5 

 sound seeds per cone. Wakeley et al. (1966) tried a simple method for 

 mass-producing this hybrid by dusting a mixture of P. elliottii pollen 

 in large quantities on unbagged P. eohinata flowers. An average of 10.7 

 percent of the seedlings resulting from such mass-pollination showed 

 definite evidence of hybridity. They concluded that, if mass-pollinations 

 were carried out only the high hybrid-yielding trees (which yielded 

 20 percent hybrid progeny) , the technique would be more economical than 

 controlled pollination. 



Pinus palustris Mill, x P. elliottii hybrids planted in central 

 Louisiana are demonstrating desirable characteristics of both parent 

 species. They closely resemble P. palustris in form and branching habits, 

 but start height growth immediately and grow almost as fast as P. 

 elliottii. They appear less susceptible than their parents to the brown 

 spot needle blight of P. palustris and the fusiform rust of P. elliottii 

 (Derr, 1966). 



MASS PRODUCTION OF RUST -RE SI ST ANT PLANTING STOCK 



The eventual commercial production of planting stock may be by any 



or all of the following procedures: (1) Single- or multiple-cross hybrids 



(2) varietal blends; (3) multilineal varieties; (4) synthetic varieties; 

 (5) multiclonal varieties. 



SINGLE- OR MULTIPLE-CROSS HYBRIDS 



The evidence from crop breeding indicates that the best F]_ single- 

 or double-cross hybrids are generally superior to synthetic varieties. 

 The use of synthetics stemmed from the difficulties and cost of producing 

 hybrid seed or from the susceptibility of pure lines or Fi hybrids to 

 inimical environmental factors, particularly disease and insect infesta- 

 tion. 



The availability of practical methods for the production and diploidi- 

 zation of haploids and/or of natural or induced cytoplasmic male sterility 

 in forest trees would open new avenues for improvement breeding through 

 intra- and interspecific hybridization. For additional populational 

 buffering, hybrids could be used in varietal seed blends or in multi- 

 clonal hybrid varieties. 



