BREEDING RUST RESISTANT TREES: MODERATOR'S SUMMARY 61" 



t\HAT IS THE CORRECT BREEDING MODEL FOR INCORPORATING 

 RUST RESISTANCE INTO PINES? 



The long period between germination and harvest of a forest species 

 such as Pinus monticola Dougl. or ?. strobus L. (i.e., 60 to 100 years) 

 is one of the greatest obstacles confronting the forest tree breeder. 

 Currently he has virtually no experimental data to determine the relation- 

 ship of seedling or early sapling growth rate and form to the growth rate 

 and form of the tree during the latter part of its si lvi cultural cycle. 

 In the case of breeding for rust resistance he is in an even worse 

 dilemma because of the difficulty of determining the effective longevity 

 of the resistance. Kill the rust resistance he is incorporating into his 

 breeding material today lose its effectiveness before the tree reaches 

 merchantable age? Or, will it provide adequate protection through the 

 current cutting cycle and be transmitted effectively through its seed to 

 protect the progeny for several forthcoming generations, or perhaps 

 indefinitely? If the source and level of resistance that has been chosen 

 is adequate, how can it be incorporated most rapidly and utilized most 

 effectively to provide the best and most stable protection from rust over 

 the longest possible period of time? These are but a few of the many 

 imponderables that confront the forest tree breeder. 



THE >DDEL FOR BREEDING RUST RESISTANT FOREST TREES 



I wholeheartedly agree with the points of view that have been 

 expressed during this symposium (and in recent publications) by Bingham, 

 Schreiner, Heimburger, Hoff, McDonald, Hattemer, Patton, Zobel, Kinloch 

 and others that successful breeding for rust resistance in long-lived 

 forest trees should be based on the accumulation of polygenes in a 

 general (uniform or horizontal) type of resistance. Dr. J. C. Zadok's 

 statement in these proceedings that, "The use of polygenes in breeding 

 for partial resistance is 'nature's own method' and it is possibly the 

 fastest and safest way to improvement," is certainly apropos. 



Frankly, however, I am both confused and considerably concerned 

 with the schizophrenic nature of the philosophy supporting many of the 

 forest breeding programs. Many pay lip service to using general (poly- 

 genic horizontal) resistance (Caldwell, 1968), but simultaneously look 

 over their shoulder and flirt with the use of specific, oligogenic, 

 monogenic, hypersensitivity, or vertical resistance. There seems to 

 prevail a fear that although general (horizontal) resistance is the 

 rational approach, it may not be adequate or may not persist and that 

 adequate protection may yet be found in specific resistance. Presenta- 

 tions and discussions at this symposium have evolved largely around 

 specific resistance, although it has failed miserably to provide stable, 

 long-lasting resistance in wheat, oats, and flax (van der Plank, 1968). 

 I can personally attest to the futility of using it in forest tree 

 breeding programs, where it may be even less dependable. Don't emulate 

 the wheat, oat, or flax breeders, but instead, as I have previously 

 suggested, adopt the methods of the maize breeders (Borlaug, 1958 and 

 1966). 



Flor's hypothesis, based on the existence of a gene-for-gene rela- 

 tionship between the host and pathogen, appears to be valid for the 

 specific rust resistance in self-pollinated plants. It does not 

 necessarily follow, however, that this relationship holds for the 

 quantitatively inherited polygenic uniform resistance in allogamous 



