CONIFERS VS. CROP PLANTS AS BREEDING MATERIAL 27 



the world do you have the virulence to attack this variety. The important 

 thing is to find out if any place in the world there is a combination 

 that is virulent. If it occurs some place then you can be quite sure, 

 once the resistance gene is widely used, the virulent race will come 

 here. The further you are away from the virulence, the better off you 

 are. Even though you can find virulence some place it's very dangerous 

 to rely on a major gene too extensively. Now, concerning pine trees, 

 you have a great advantage because with pine you are dealing with the 

 haploid stage of the white pine blister rust fungus. If you have a 

 sporidia floating around, and there is one having the proper virulence 

 gene, you can't cover it up by a dominant gene for avirulence. In pine 

 the rust mycelium is haploid, and you will find a recessive virulence 

 gene immediately. Thus testing a resistant pine against populations of 

 Cronartium ribicola all over the world will accomplish even more than 

 the testing we do in cereals. Virulences in cereal rusts can be covered 

 up in the dicaryotic stage of the fungus. 



BINGHAM: To return a moment to the "major-gene monoculture" problem, 

 Mr. Barnes, Dr. Kinloch, and their administrators have another solution. 

 They recognize that this major resistance gene is a step forward, and 

 also its danger. They are proposing to use it on a limited scale in 

 plantations where the resistant white pines would be mixed with trees of 

 other species. 



CALLAHAM: This brings me to a topic that may not have been brought 

 out enough in Dr. Duffield's presentation. What is a monoculture in 

 Forestry? Typically, a monoculture in wheat is the planting of almost 

 pure fields of one strain from the center of the Canadian plains to the 

 highlands of Mexico. That kind of a monoculture, I think, never would 

 exist in forestry for many reasons that Dr. Duf field gave. Forestry 

 through its harvesting and regeneration processes moves from one small 

 piece of land to another. Forestry rarely creates at one time the condi- 

 tions for epiphytotics that would exist in a wheat monoculture. Here is 

 another point. Presumably we are going to build dynamism into our tree 

 breeding process. We will constantly be generating new varieties having 

 different combinations of resistance factors. We can create different 

 mixtures of resistant entities within the type that is planted. On any 

 one mountainside, as a rotation occurs, foresters will create a spectrum 

 of new and different resistant types. Other species may be intermingled 

 as Duf field brought up. We really need to focus our thinking on what is 

 a monoculture in forestry in contrast to what is a inonoculture in 

 agriculture. 



DUFFIELD: I believe Dr. Callaham is correct in his suggestion that 

 the term monoculture has different meanings in agriculture and in forestry, 

 Foresters have generally been concerned with the effects of monoculture 

 on soil properties and less concerned with pathological effects. We may, 

 in the future, develop a more agronomically-oriented attitude. 



BORLAUG: I would like to make two comments. One, I think this major 

 gene that was discussed a minute ago is something that is useful and 

 something that should be used. I think that it should be used in the 

 context of using it in combination with the other kinds of genes you have 

 already identified. The problem then is that when you superimpose one 

 of these major genes--almost always a dominant--it will mask the effect 

 of others. Therefore the problem becomes one of identifying or knowing 

 when you still have the polygenic type of resistance underneath. My 

 second comment concerns the remarks Dr. Callaham just made. I think you 



