576 ERNST J. SCHREINER 



"The use of recurrent backcrossing for improving crops in 

 characters dependent on numerous genes is limited only by the 

 ability of the plant breeder to select for a worthwhile 

 intensity of the character. This is the chief limiation in 

 dealing with such characters, regardless of the method employed. 

 In order not to dissipate the desired genes, each backcross 

 must be followed by rigid selection." 



SOURCES OF INTRASPECIFIC RESISTANCE 



White Pines 



The current breeding programs (listed by Bingham et al. , 1969) aimed 

 at mass production of partially blister rust resistant types of Pinus 

 strobus, P. monticola, and P. lambertiana Dougl . are based on sound evi- 

 dence for sufficient natural resistance to warrant such programs. On 

 the basis of their research with P. montioola , Bingham et al. (1969) have 

 predicted an expected gain of 4.3 to 7.1 percent, 8.5 to 19.9 percent, 

 and 9.9 to 23.7 percent, from Stage A, B, and C clonal seed orchards, 

 respectively. 



Southern Pines 



Barber (1966) found highly significant differences in fusiform rust 

 infection among half-sib families from three Pinus taeda L. stands in 

 Georgia. There was a ninefold increase in rust-free trees in the best 

 progeny as compared with the poorest; some of the fastest growing families 

 were among the more resistant to fusiform rust. Jewell and Mallett (1967) 

 have concluded from controlled pollinations among and between rust-free 

 and rust-infected selection of P. elliottii Engelm. : 



"...that resistance and susceptibility are under strong 

 genetic control and are transmissable . Thus geneticists can 

 plan a breeding program for general tree improvement and can 

 implement it by selecting rust-free parents..." 



INTERSPECIFIC HYBRIDIZATION 



White Pines 



Bingham (1961) has made a strong case for intraspecific breeding as 

 a rapid and safe means to meet the immediate need for resistant planting 

 stock. But he adds that, ultimately, it probably will be necessary to 

 incorporate resistance factors from other species, and that "A common- 

 sense approach is to carry separate lines of resistant inter-species 

 hybrids, meanwhile selecting for adaptations while slowly incorporating 

 them in the over-all breeding scheme." Callaham (1962) has suggested 

 that hybrids of P. strobus x Pinus wallichiana A.B. Jacks, should be 

 subjected to further investigation, and that the rust resistance of 

 hybrids involving P. montioola, P. strobus, and P. wallichiana could be 

 enhanced by using parents selected for good general combining ability 

 for resistance. Heimburger (1962) and Patton (1966) have shown that it 

 is possible to introduce into Pinus strobus factors for blister rust 

 resistance from less-susceptible related species such as P. wallichiana, 

 Pinus parviflora Sieb. $ Zucc, and Pinus peuce Griseb. 



