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BREEDING RUST RESISTANT TREES: MODERATOR'S SUMMARY 623 



I have been greatly impressed with the progress that already has 

 been made by R. T. Bingham and colleagues in breeding blister rust- 

 resistant Pinus monticola through intraspecific crossing (Bingham, 1966) . 

 Similar progress has been achieved by C. Heimburger, and A. J. Riker 

 and R. F. Patton in breeding for blister rust resistance in P. strobus 

 by the intraspecific approach. 



During the past 4 days I have been fascinated as the story of high 

 levels of resistance to blister rust were reported by S0egaard, Gremmen, 

 Bakshi, Heimburger and Bingham in Pinus sibirica Du Tour; P. koraiensis 

 Sieb. & Zucc. , P. armandii Franch., P. parviflora Sieb. S luce, P. 

 wallichiana Jacks, (syn. P. griffithii McClell.), P. peuce Griseb. and 

 P. cembra L. 



Most interesting of all to me, however, are your reports that, in 

 P. monticola and P. strobus, about 1 tree in 20,000 and 1 tree in 10,000, 

 respectively, have survived the onslaught of the blister rust epidemics. 

 Most of the phenotypically resistant trees that have been studied in 

 detail have proved to be genotypically resistant, not escapes. Con- 

 trolled crossing between genotypically resistant trees of P. monticola 

 followed by progeny testing of the seedlings, has definitely established 

 that an additive polygenic type of general resistance is involved. 



All individuals in Castanea dentata and apparently also in Ulmus 

 americana populations were uniformly susceptible to the introduced 

 pathogens Endothia parasitica and Ceratocystis ulmi respectively, and 

 both host species face extincion. This appears to be good circumstantial 

 evidence that neither American chestnut nor American elm had ever been 

 in contact with the corresponding pathogens before their recent introduc- 

 tions. By contrast, genes for resistance to C. ribioola are present in 

 both P. monticola and P. strobus. WHY THE DIFFERENCE? Yesterday I made 

 a statement that it appears to me you are dealing with "fossil" poly- 

 genes for resistance to C. ribioola in both P. monticola and P. strobus. 

 How else can one explain the existence of polygenic resistance in these 

 two species? 



The genes for resistance to C. ribicola in P. monticola and P. 

 strobus, although originally present at a low frequency and widely 

 dispersed, are being brought back together successfully and increased 

 in frequency through use of selection pressure of the pathogen in progeny 

 tests following crossing. The resistance may be functional against the 

 entire range of pathogenicity of C. ribicola currently present in 

 Western North .America. I hypothesize that these are "fossil genes" that 

 have persisted in P. monticola and P. strobus since geologic time. 

 These genes would not be present in these North American pines unless 

 there had been previous contact between C. ribicola and P. monticola and 

 P. strobus. There is no way to predict accurately how far back in 

 geologic time this host-parasite relationship existed. 



Conifers are very old, but the fossil evidence supporting their 

 antiquity is fragmented. There is evidence, however, from petrified 

 forest fossils in several parts of the world, that conifers were well 



^Editor's note: Another explanation might be gene (pollen) exchange 

 between the "resistant" Eurasian white pines and the progenitors of the 

 2 present North American species, even though they had not come into 

 d.irect contact with the rust. 



