22 JOHN W. DUFFIELD 



diseases. When the question of rust races is attacked, the relatively 

 long cycle of the pathogen will constitute a serious technical problem. 



So far, I have been discussing some of the inherent features of trees 

 in general and conifers in particular. Equally important are some of the 

 characteristics of forestry which affect both disease processes and the 

 constraints under which the breeder operates. An article of faith which 

 underlies many of the attitudes and practices of foresters is the concept 

 of sustained yield. While it must be admitted, as Duerr (1967) has pointed 

 out, that adherence to this doctrine may induce excessive rigidity in the 

 thinking of foresters, it is also evident that the concept is useful as a 

 safeguard against expediency and irresponsibility. In any event, foresters 

 are usually committed to the notion that the areas in their care are to be 

 maintained as forest over an indefinitely long period. This notion imparts 

 certain characteristics to forest management. 



The concept of growing stock is fundamental to forest management. 

 It means that the cambium of the trees composing the forest is the essen- 

 tial productive facility or capital. Since this cambium does not become 

 practically effective in producing wood in significant volume or quality 

 until the trees it envelops have reached appreciable size and age, growing 

 stock cannot be built up quickly, nor can sudden changes in the nature of 

 the growing stock be made. The contrasts with the nature of most agronomic 

 crops are obvious. Forest management exhibits much greater stability-- 

 or inertia--and is therefore more seriously disrupted by killing diseases 

 and other destructive agents than are many sorts of agriculture. 



The inertia of forest management is reinforced by a tendency to rely 

 on so-called natural regeneration in many instances. Where such reliance 

 is total or predominant, the scope for genetic intervention is virtually 

 nil, although skillful conservative silviculture may avoid dysgenic changes 

 in the forest. Obviously naturally regenerated forest may prove a useful 

 starting point for a program of resistance breeding, but application of 

 the results depends on artificial establisment of the new crop. 



Pure cultures of trees, as of any other organisms, have various ways 

 of bringing about their replacement by something else. The practical 

 recognition of this in agriculture is crop rotation. A common solution 

 in forestry is the impure culture or the mixed stand. This has interest- 

 ing consequences for the relationship between host species and disease. 

 On the one hand, it tends to maintain a constant host population or sub- 

 strate for the disease. On the other, it dilutes the impact of a single 

 disease on the forest as a whole. 



Forests may have high esthetic values and the public has increasingly 

 high critical standards of forest esthetics. This is not to imply that 

 many farm landscapes are not beautiful. However, forests are often 

 closely associated, in fact and in the mind's eye, with mountains and with 

 water, and this association raises the expectations and heightens the 

 sensitivity of the onlooker. Maintenance of the esthetic values of the 

 forest is also associated with the features of forestry I have just 

 enumerated — continuity , diversity, and something called naturalness. The 

 esthetic aspect of forests has its interesting implications for the resis- 

 tance breeder. So-called artificial control programs, especially those 

 involving aerial applications of chemicals, are under increasing fire from 

 the public. The ribes eradicator, protecting white pines in our National 

 Parks from blister rust, has had to work stealthily and with only partial 

 approval of the park visitors. His successor, attempting to introduce 

 resistant trees into so-called natural habitats, may anticipate similar 

 problems. 



