DANGEROUS INTERNATIONAL FOREST TREE DISEASES 117 



Principles of Forest Disease Control 



J. S. Boyce 



Professor Emeritus of Forest Pathology, Yale University, 

 New Haven, Connecticut, U.S.A. 



I feel presumptuous in discussing the principles of forest disease 

 control before this group since most of you are specialists in the field, 

 and therefore anything I can say will sound trite. However, there 

 will probably be some disagreement with my ideas, and reasoned 

 disagreement is invaluable for the complete exploration of any topic. 



There is often such a difference in the effect of diseases caused by 

 native pathogens on forests that control measures differ widely. For 

 native diseases these measures are largely indirect — that is, applied 

 as a part of Foutine silvicultural practice: whereas, for introduced 

 diseases, direct measures must often be used — that is, special opera- 

 tions solely against the disease. TVhenever possible, indirect meas- 

 ures should be employed. Diseases caused by native pathogens do 

 not threaten the commercial extinction of a native tree species, but 

 diseases caused by introduced fungi may eliminate, and in certain 

 cases have eliminated, important timber species. Consequently, there 

 is usually far greater urgency to control diseases caused by introduced 

 pathogens than to check those caused by native pathogens. 



Control measures are limited since forest stands in general have a 

 relatively low value considering the years required to produce them, 

 therefore expenditures for disease control must be modest. Thus, 

 the intensive methods of disease control that are routine in agri- 

 culture and horticulture are possible only occasionally in forestry — 

 the exception being nurseries where intensive control measures are 

 standard practice. Insofar as possible, control should be part of regu- 

 lar silvicultural practice. 



Certain precepts should be pursued in order to minimize disease. 

 Each of us can cite exceptions to these precepts and, because of con- 

 ditions in some forest regions, it may seem necessary to disregard 

 them. When this is done, it should be with realization of increased 

 expenditures that may have to be made for disease control. Some of 

 these precepts follow: 



_ Seedling stands are better than sprout stands. Natural regenera- 

 tion is better than artificial, particularly planting. Uneven-aged 

 stands are better than even-aged. The selection system is better than 

 clear cutting. Mixed stands are preferable to pure stands, and mix- 

 tures of conifers and hardwoods are especially desirable. Natural 

 stand composition should be followed as far as possible. Of course in 

 certain forests nature's method is pure stands which should be ac- 

 cepted. Good sites are essential — no amount of care can produce a 

 satisfactory stand on an unsatisfactory site. Native species are safer 

 than exotics, because exotics are more liable to injury when conditions 

 become critical. Because of the high risk of bringing in new patho- 

 gens, exotics should be introduced as seed only. The seed must be of 

 proper origin both as to location and character of the mother trees. 



Our safeguard against introduced pathogens is quarantines which 

 admittedly are measures of delay and not measures of exclusion. 



