INSECT PESTS AND DISEASES. 



Some of the attributed causes are : 

 1. Sadden checks to the vegetation of the tree, especi- 

 ally in spring and the early part of summer. 



FIG. 51. CANKER IN APPLES, SHOWING THE PRESENCE OF A FUNGUS, 

 NECTRIA DITISSIMA. 



2. Derangements of the flow of sap from vicissitudes 

 of heat and cold, as well as of moisture and dryness. 



3. Unskilful and severe pruning. 



4. Vitiation of the sap by deleterious substances in the 

 soil or subsoil, 



