IHtUKCl lLrtL d/w. 



RESISTANCE OF WHITE PINES IN EASTERN NORTH .AMERICA 269 



P. szrc'cus . --The results of one series of progeny tests with selected 

 clones from Pointe Platon are shown in Table 8. The crosses were made to 

 obtain an F2 generation of resistant seedlings for further breeding work 

 and to evaluate 9 clones selected for resistance under conditions of heavy 

 infection in the field and confirmed in our tests with grafts. As can be 

 seen, the experiment was only partially successful. Only one clone showed 

 promise of producing an ¥2 generation while the other 8 clones yielded 

 seedlings that were eliminated by blister rust 5 to 6 years after inocula- 

 tion. Most of these were populations with unstable resistance, i.e., 

 they continued to segregate susceptible seedlings up to 4 years before 

 being eliminated by blister rust. Most of our other crosses within P. 

 strobus are as unsatisfactory from a resistance breeding standpoint as 

 those mentioned above. Selected materials usually have some resistance 

 for a year or two after inoculation, but then the populations are eliminated 

 by rust or segregate susceptible seedlings in succeeding years, leaving 

 only very few plants for further work. 



This indicates, perhaps, that the inoculation method has not been 

 adequate to evaluate resistance and a more intensive method for infection 

 is needed. It is probably also an expression of an interaction of resis- 

 tance and environment. Mother possible reason for the segregating seed- 

 ling populations so often observed in screening P. strobus for resistance 

 is simply the lack of certain resistance genes in this species. If this 

 is the case, the introduction of such genes from other species may be the 

 only way to form a reasonably realistic breeding program with P. strobus. 



LITERATURE CITED 



Childs, T. W. , and J. L. Bedwell. 1948. Susceptibility of some white 



pine species to Cronartium ribicola in the Pacific Northwest. J. 



Forest. 47: 595-599. 

 Heimburger, C. 1956. Blister rust resistance in white pine, p. 6-13. In 



Proc. Northeast Tree Improvement Conf., 1955. 

 Hirt, R. R. 1940. Relative susceptibility to Cronartium ribicola of 



5-needled pines planted in the East. J. Forest. 58: 932-937. 

 Hirt, R. R. 1948. Evidence of resistance to blister rust by eastern 



white pine growing in the Northeast. J. Forest. 46: 911-913. 

 Patton, R. F. 1966. Interspecific hybridization in breeding for white 



pine blister rust resistance, p. 367-376. In H. D. Gerhold et at. 



(ed.), Breeding pest-resistant trees. Pergamon Press, Oxford. 505 p. 

 Riker, A. J., and T. F. Kouba. 1940. White pine selected in blister rust 



areas. Phytopathology 30: 20. (Abstr.) 

 Spaulding, P. 1925. A partial explanation of the relative susceptibility 



of the more important American white pines to the white pine blister 



rust. Phytopathology 15: 591-597. 



FLOOR DISCUSSION 



Panel leader Bingham withheld discussion of this paper until after 

 the closely related paper by Bingham, immediately following. 



