PERSISTENCE OF RUST RESISTANCE 565 



nature of the resistance involved and also means a more elegant, albeit 

 more costly, way of finding resistant material than imposing heavy infec- 

 tion in only one test. 



As it is hazardous to speculate on the future of resistant varieties, 

 no predictions are attempted. The resistance breeders continuously 

 observe their field tests in order to derive offspring-parent correlations, 

 to keep an eye on the behavior of the disease, and to study the mode of 

 increase in mortality. It might not be too alarming if at some later 

 age a variety shows a sudden increase in mortality. In clonal seed 

 orchards, for instance, it often can be observed that a few clones are 

 killed by some endemic fungus that formerly deserved little interest. 

 Whether an effect of counteraction in the pathogen is involved may be 

 hard to prove. So, the recommendation may be given to study everything 

 new right after it happens, not only to keep up with new virulence but 

 to be one tree generation ahead. It must likewise be true with a sympatric 

 and an introduced obligate parasite that, because of the length of the 

 tree life (Heybroek, 1969) , mass-growing, particularly of interim planting 

 stock in pure stands, is self-destructive. One may argue whether also 

 small experimental plantations exert troublesome selection pressure on 

 the parasite population, but certainly the early release of varieties with 

 anything but a broad basis of resistance may endanger the success of at 

 least one generation of improvement. 



CONCLUSION 



I am impressed by the progress made in the project underway at 

 Moscow. But I would pose these questions to Mr. Bingham concerning the 

 outlook of this project. The 1969 paper (Bingham et at. , 1969) that was 

 submitted in 1967 pointed out that the superiority of certain candidate 

 offspring was likely to be underestimated. Does some new information 

 exist on how much progress was obtained by the operations demonstrated 

 here? How close was the eventual selection goal of much less than 100% 

 healthy trees approached? 



LITERATURE CITED 



Bingham, R. T., R. J. Olson, W. A. Becker, and M. A. Marsden. 1969. 



Breeding blister rust resistant western white pine. V. Estimates of 



heritability, combining ability, and genetic advance based on tester 



matings. Silvae Genet. 18: 28-38. 

 Falconer, D. S. 1961. Introduction to quantitative genetics. Robert 



MacLehose and Co., Ltd., Glasgow. 365 p. 

 Heimburger, C. 1962. Breeding for disease resistance in forest trees. 



Forest. Chron. 38: 355-362. 

 Heybroek, H. 1969. Three aspects of breeding trees for disease resis- 

 tance. FAO 2nd World Consultation on Forest Tree Breeding Docum. 



FO-FTB-69-5/4, 12 p. 

 Schtltt, P. 1964. Lophodermium-Bef all der Kiefer in Abhangigkeit von 



Herkunft und Anbauort. Forstw. Cbl. 83: 140-165. 

 van der Plank, J. E. 1968. Disease resistance in plants. Academic 



Press, New York. 206 p. 

 Walker, J. C. 1966. The role of pest resistance in new varieties, p. 



219-242. In K. J. Frey (ed.), Plant breeding. Iowa State Univ. Press 



Ames . 430 p. 



