12 



THE ROYAL SOCIETY OF CANADA 

 Leaf Roll of Potatoes 



The amount of loss from leaf roll is indicated in the following 

 table. It may be taken as a general rule that the reduction in yield 

 is about 65 per cent, although it may greatly exceed this. 



(4) Before passing on to the problems of prevention I cannot 

 refrain from making brief reference to forest pathology, a branch of 

 pathology replete with problems, few of which have even been ap- 

 proached in America. The life histories of many timber diseases have 

 not yet been worked out, almost nothing is known of their ecological 

 relations, there are few data on their distribution or the extent of their 

 destructiveness and little done towards making practical application of 

 the knowledge we have in hand. That the losses from native fungous 

 diseases alone in the forests are enormous is beyond question, aside 

 altogether from the striking devastations wrought by such introduced 

 plagues as the chestnut blight. Heart rots and root rots in particular, 

 the most important without exception of all timber enemies in this 

 part of America, are widely prevalent; and in some of the forested 

 areas of Canada at the present time it is reasonably certain that the 

 annual increment is more than offset by the inroads of these fungous 

 diseases. Moreover, they bear a direct relationship to forest fires in 

 that the vast amount of highly combustible debris on the floor of the 

 forest consists of the remains of trees that have fallen a prey to dis- 

 ease. The time is fast approaching when real forestry will have 

 to be practised in America, and when that time comes, the health of 

 the forest will be the central consideration. Meanwhile pathological 

 surveys of our forest resources and intensive investigations on the more 

 serious forest diseases will be a wise preparation. 



(5) Turning to problems of disease control it is scarcely necessary 

 to repeat that herein lies the heart of the entire subject. Nor is it 

 necessary to further point out the established economic success of the 

 science. Its applications are indispensable and there is no more 

 likelihood of their abandonment by the practices of agriculture and 



