INTRODUCTION 21 



this is the case we should not at once stop the investigation 

 when a cause of disease has been discovered. Very often we 

 encounter, e.g. in the low lands of North Germany, devastated 

 woods of Scotch pines, in which many trees have been killed by 

 Trametes radiciperda. More exact investigation, however, often 

 results in the discovery that in the same wood, in consequence 

 of insufficient circulation of air in the soil, root-rotting is much 

 more destructive even than the root-parasite. 



Only the most careful research, supported by thorough know- 

 ledge of the forms of disease, which are so numerous and 

 varied, can protect us against error. 



