Forest Sanitation 487 



the United States seems to hinge upon the resistance of certain 

 foreign varieties of chestnut to the ravages of the chestnut bark 

 fungus now destroying the native chestnut in the East. 



We have shown that the external environment affects the well 

 being of the plant and the difficulty in making a correct diagnosis 

 of a diseased condition. More and more is the doctrine of the 

 "predisposition" to disease advanced. This doctrine holds that 

 the infection of the host plant by a fungus is in more cases pos- 

 sible only when the plant is susceptible to it. Considering the 

 multifold conditions under which most plants exist, the selec- 

 tion of the weakening factor by which the organization may be 

 strengthened against a disease is difficult. However, this pro- 

 phylactic procedure of increasing the resistance of the species 

 or race is of some practical importance, and where climatic con- 

 ditions are concerned the silvicultural practice may be so varied 

 as to bring good results. 



The control of heteroecious forest tree fungi in most cases 

 hinges on the elimination of the alternate host and this brings us 

 to the most important part of a prophylactic treatment of 

 plant disease, viz., obtaining a knowledge of the complete life his- 

 tory of the fungus itself. One noted plant pathologist has stated 

 that every fungus has in its development "an Achilles tendon." 

 This means, of course, knowing in what period of development to 

 strike the fungus in order to obtain the most good. To illustrate 

 the nature of heteroecious forest tree fungi, there are in our 

 northwestern forests some half dozen diseases which produce 

 abnormal swellings of trunk and branches. Occasionally a dense 

 matted branching effect is produced known as "witches brooms." 

 The result of these is to stunt and eventually kill the host. A 

 number of these fungi have an alternate stage on some other 

 plant and without it are unable to infect anew other forest trees. 

 The value of knowing the alternate host, and as yet we know but 

 few of them, is evident. It is like the fruit grower who cut 

 down a fine stand of pear trees for the purpose, as he thought, of 

 eradicating a heteroecious leaf disease, and allowed the solitary 

 juniper tree, the alternate host of the fungus and the cause of 

 the trouble, to remain. 



In the care of the forest nursery it is important to successfully 

 bring young seedlings through certain growth periods because 

 these are periods of greatest susceptibility to certain virulent 



