CHAPTER X 



CENERAL SUMMARY 



Whetzel in his History of Phytopafhologij (1918) says that 

 ' modem pathologists may be divided, for the most part, 

 into two philosophic schools, the pafhogenefisfs and the 

 predisjiosifionisis '. Most of as object to being labelled, and, 

 for our part, we do not call ourselves either pathogenetist 

 or predispositionist. Nevertheless the terms have their 

 value. Every disease described in this book is caused by 

 some specific fungus, and this fungus is considered as the 

 primary factor in the etiology of the disease. With some 

 of the diseases secondary factors have also to be taken into 

 account, such as sylvicultural conditions, which make for 

 the good or bad health of the tree as a whole. The patho- 

 genetist emphasizes the primary factors, the predispositionist 

 the secondary factors. But whether we are to be one or 

 the other depends entirely on the disease which we are 

 considering. The general health or tone of the tree plays 

 no ])aTt in the incidence of diseases caused by the rusts 

 described in the last chapter. If sporidia 'of a suitable 

 species of Melampsora or Melamjpsoridimn blow on to leaves 

 of larch under suitable conditions for germination, infection 

 is almost certain, however healthy or unhealthy the larch 

 may be. The canker fungus, on the other hand, is largely 

 dependent on finding trees which are not growing with full 

 vigour. This fungus is always with us on the dead branches 

 of healthy trees as much as on those of unhealthy trees. 

 Whether or no it is to become parasitic depends on its 

 opportunities for infection, and this is chiefly determined 

 ])y the vigour of the trees. With th(^ rust diseases, then, 

 we are pathogenetist ; with the canker, ]iredis])ositionist. 

 With most other diseases we ai-e somewluMc betwixt and 

 between. 



