118 U.S. DEPT. OF AGRICULTURE MISC. PUB. 939 



Nevertheless, delay is more important in forestry than in agriculture 

 because of the long time necessary to produce a crop of trees and to 

 work out control methods when disease does appear. 



The control or extermination of a pathogen by another pathogen — 

 that is, biological control — theoretically should be the cheapest and 

 most effective method of dealing with pathogenic diseases. Yet so far 

 there has been no instance of any worthwhile success in this field 

 against fungi and related organisms, and nothing now appears promis- 

 ing for the future. 



The development of resistant trees by selection or hybridization, 

 when this can be accomplished, is probably the best method for con- 

 trolling disease. The limitations of the method are the necessity for 

 developing several resistant hybrids or selections and the long time this 

 requires. This means that in dealing with a disease caused by a 

 virulent pathogen a successful method of control must be developed 

 quickly so the affected tree can be perpetuated until such time as suffi- 

 cient resistant individuals can be obtained. 



Finally a new and promising method of control is being developed; 

 namely, the use of systemic fungicides. Lesions of white pine blister 

 rust on western white pine trees have been reported to be inactivated 

 within 18 to 24 months after the boles of the trees have been sprayed 

 to saturation from ground level to a height of 5 feet with 150 parts 

 per million of cycloheximide in fuel oil. This mixture also shows 

 promise as a foliage spray against blister rust. The use of systemics 

 may bring revolutionary changes in control methods in the next decade 

 or so. 



