SPECIFICITY IN PLANT DISEASE 39 



ZADOKS : In general, I conform with your ideas. 



PERSON: I think you would be in trouble, Bill, if you had to 

 accommodate this kind of gene whose action was to prevent infection so 

 that the fortunate tree with this gene just never was infected. For one 

 day a race of the rust could come along that could handle this gene and 

 infect. How would you then fit this into your idea of the aegricorpus? 



LOEGERING: Well, we might do a little juggling at that time, if 

 you find the situation occurring. 



KINLOCH: I wonder how useful the gene-for-gene concept is in the 

 case of an exotic disease on a native species. 



LOEGERING: The gene-for-gene concept is only valuable for purposes 

 of thinking and I tried to emphasize this. The gene-for-gene concept 

 does not change the methods that you use in doing your plant breeding. 

 The methods you use to handle your pathogens, and so on. But, it is 

 useful in the analysis of information and that sort of thing. It makes 

 your observations so much more meaningful. I want to emphasize very 

 strongly that what we are dealing with here is a revision of the way we 

 look at disease. That's what it amounts to and it is this revision of 

 point of view that the gene-for-gene concept has given us. But, the 

 gene-for-gene concept as a method of plant breeding hasn't added much. 

 It has made it easier to understand what's happening and that is why I 

 limited the gene-for-gene concept. To me it's very helpful. Now, you 

 get into the exotic diseases. Well, maybe there are no gene-for-gene 

 relationships because of the things you mentioned. That doesn't make 

 any difference in the whole system, but if there are any you see them. 

 I'd like to comment on something that Dr. Duffield had said. I was going 

 to mention this in my paper but I didn't get over half of what I was 

 going to say. The question of where the blister rust came from and where 

 Pinus montioola Dougl . came from is important. At the present time we 

 have a world survey underway in order to determine something of a basic 

 nature. Is it or could it be true that the center of origin of your host 

 is in region A, the center of origin of your pathogen is in region B and 

 the center of origin of the disease is where regions A and B overlap? I 

 would suggest that you might apply this point of view to thinking about 

 centers of origin. This is only one way of thinking, but to think about 

 things this way comes from an understanding of the gene-for-gene concept 

 itself. The origin of the disease might not be in either the center of 

 origin of the pathogen or the host. 



MCDONALD: We have a rather unusual situation with white pine 

 blister rust in that resistance mechanisms can be separated in time and 

 space. This relates to your infinite selection idea. We have the possi- 

 bility of a resistance factor in a secondary needle, in the stem, or in 

 the primary needle or traumatic simple leaf as it is sometimes called. 

 Since there are three separate infection courts, resistance mechanisms can 

 be bypassed. This means that the idea of infinite selection must be 

 expanded to include several levels. That is a type or race of Cvonavtium 

 that may be avirulent on the secondary needle could. establish an infec- 

 tion and produce aeciospores on that plant by entering through primary 

 needles. It's just one more thing we have to consider. 



LOEGERING: This was the subject of a rather lengthy discussion last 

 night regarding smuts because you determine what you want to know by the 

 percent of smutting. You don't know what happens in infected plants until 



