536 RAYMOND J. HOFF AND GERAL I. MCDONALD 



HOFF: I would like to add a little here. The percentage of yellow 

 spots was 60 whereas the percentage of red spots was 40 in the 1964 

 progeny. We got the same proportion in the 1966 progeny tests. 



MCDONALD: That brings to mind another observation. When we classify 

 the individual seedlings, we observe seedlings with no spots, seedlings 

 with only red spots, seedlings with both red and yellow spots, and seed- 

 lings with only yellow spots. We looked at the frequency of needle 

 lesions on the seedlings with only red spots and it averaged four lesions 

 per 25 inches of needles. The frequency of lesions on trees with only 

 yellow spots averaged six lesions per 25 inches, and seedlings with both 

 types of spots averaged 10 lesions per 25 inches. 



KINLOCH: Dr. Hoff, you mentioned that the endemic rusts are appar- 

 ently living in a state of balanced symbiosis in which biological damage 

 was not too severe--just economic damage. I take exception to this. I 

 think the biological damage is very severe, and I think you only have to 

 look at Harry Powers' slide this morning to be convinced. The mortality 

 and selection pressure against the host species is quite severe in many 

 of these rusts, especially in juvenile stages of the host, and therefore 

 balanced symbiosis may not come from this uniform resistance that you 

 were talking about, but rather from other specific resistance factors 

 carried in the population at intermediate frequencies. At least this is 

 an equally plausible hypothesis, I think. Secondly, I'd like to make 

 another comment on your proposition that species in the same genus may 

 carry the same resistance factors. I think it's worthy of note that red 

 pine, as Harry Powers points out, is apparently resistant to all rusts, 

 both endemic and exotic. Yet, presumably, the selection pressure has been 

 on it for millenia. You can make a similar case for short-leaf pine 

 resistance against fusiform rust in the South. These species, although 

 closely related to other endemic species, have maintained a very high and 

 stable resistance. I think this should make one pause before a priori 

 assuming specific resistance will inevitably become labile and break 

 down. 



HOFF: First, selection in a balanced symbiotic system would select 

 for resistance factors that could be classified as either specific or 

 uniform. We didn't want to imply that natural systems are controlled 

 only by uniform resistance factors. But what we did want to do is to 

 increase its importance in our thinking. Second, balanced symbiosis is 

 based on the assumption that the balanced host parasite system is an 

 interaction in and also with a particular environment. Man has spread 

 C. fusiforme and C. querouum and their hosts around and so now these 

 diseases and especially C. fusiforme are no longer in balance. They are 

 beginning to behave just like "exotic" diseases. 



