530 RAYMOND J. HOFF AND GERAL I. MCDONALD 



Scheme 3 appears to represent the best chance for lasting success, 

 especially when the disease cycle is considered. So far, two factors 

 for resistance appear to fall into the uniform resistance category, i.e., 

 needle lesion frequency and slow canker growth. We hope there are 

 others. Because of the "vertifolia" effect, van der Plank (1963, 1968) 

 warns against selecting for uniform and differential resistance types at 

 the same time. The nature of the disease cycle of blister rust in 

 western white pine helps to circumvent this problem. Since resistance is 

 expressed sequentially at several sites separated by space and time, 

 selection can be made for both types in the same individual. For example 

 on those trees that are susceptible to either the "yellow-spot" forming 

 race or "red-spot" forming race, or both, selection could be made for 

 needle lesion frequency. Later, selection could be made for the needle- 

 spots-only traits followed by selection for corking-out, bark hyper- 

 sensitivity, or slow canker growth by stem inoculation. Simultaneous 

 selection for slow canker growth and the other bark reactions would be 

 more difficult. Selection for the corking-out reaction could be made, 

 however, since it is not often expressed until the canker is well 

 established. Thus, fungus growth rate would have been evident. 



It appears that scheme 3 could be used essentially as developed by 

 the agronomic workers. However, we must remember that agronomic programs 

 are designed to cope with short-lived, genetically homogenous, and 

 unnatural (artificial or man-maintained) systems whereas most forestry 

 programs must be designed to cope with long-lived, genetically hetero- 

 genous and natural (little or no influence by man) systems. This 

 situation poses the question, "Who should the foresters emulate--the 

 agronomist or 'Mother Nature'?" Maybe we can emulate both. 



NATURAL MODELS OF STABLE RESISTANCE 



For the sake of this discussion, let us take as fact the supposition 

 that host-parasite systems are subject to evolutionary forces. Then, we 

 should expect such systems to tend toward stability (Mode, 1958; Person, 

 1959, 1968; Person, Samborski, and Rohringer, 1962; van der Plank, 1960, 

 1963, 1968, 1969). It follows that in plant communities that have not 

 been totally disrupted by man highly stable systems would evolve. Stable 

 host-parasite systems are evident in many diseases of forest trees 

 (van der Plank, 1960) . 



The degree of balance within a host-parasite system is determined 

 by the amount of damage to the reproductive activities of both host and 

 parasite, since evolution operates through the reproductive process. 

 Furthermore, the system is composed of two separate genetic mechanisms 

 and a prolonged imbalance can end in destruction of the system. 



The North American hard pines and their corresponding indigenous 

 rusts are excellent examples of stable or balanced host-parasite systems. 

 In an extensive review of the literature on stem rusts, Peterson and 

 Jewell (1968) could cite no examples of major imbalances (epidemics) in 

 these natural systems. Although both reproductive and economic damage 

 can at times be severe, the survival of the host-parasite system is 

 secure since widespread infections appear to occur only when all require- 

 ments for infection are particularly favorable. A good example of a 

 compatible relationship, at least at the individual level, has been 

 reported by Peterson (1960) . He often found live mycelium of Peridermivm 

 harknessii Moore that was at least 200 years old in stem sections of 

 lodgepole pine (P. contovta Dougl.). 



