8 • THE ROYAL SOCIETY OF CANADA 



"It should be pointed out that last year (1917) was not an especially 

 serious one as far as diseases were concerned. No serious epidemics 

 were experienced such as it sometimes the case. The loss from black 

 stem rust of wheat in 1917 was small compared with the loss in 1916 

 when about 200,000,000 bushels of wheat were destroyed. The 

 damage caused by late blight of potatoes in 1917 was nearly 25,000,000 

 bushels but in some years the disease has been known to take many 

 times that amount of the crop." 



(1) Historically the first problems of plant pathology solved were 

 those of parasitism ; a host of them still confront us, not the least of 

 which is the nature of parasitism itself. They consist of the life 

 histories of parasitic fungi and bacteria, their origin, extent of their 

 virulence, the effect of environmental factors upon their capacities for 

 infection, the susceptibilities of their hosts to infection, the factors that 

 determine immunity and susceptibility, etc. It is true that an enor- 

 mous amount of excellent work in this field has been accomplished by 

 DeBary and the many botanists who have succeeded him; no other 

 part of plant pathology has been so well worked over. But few 

 realize the vast extent of the ground to be covered. The economic 

 plants are of so many different species, and while some parasites extend 

 their attacks to various host species, that is not generally the case. 

 Moreover, each host species is subject to many different kinds of 

 diseases. Consulting Whetzel and Hesler's Manual of Fruit Diseases, 

 written primarily not for the student but for the growers, I find 32 

 diseases for the apple described, 19 for the peach and 11 for the grape; 

 and in Rankin's Manual of Tree Diseases of the same series, 13 for the 

 maple, 19 for the oak and 23 for the pine, not including damping off 

 and various other seedling diseases. While these are the commoner 

 diseases and the ones that merit closest attention there are many 

 others of which the expert should have some knowledge, for while they 

 may be obscure, apparently unimportant, or restricted to other lands 

 some of them are endowed with the potentialities of virulent parasitism 

 given right conditions. Including saprophytes upwards of 55 kinds 

 of fungi are listed as growing on wheat, 335 on the grape, 100 on the 

 potato and 250 on the apple. Because of the unlimited scope of the 

 subject the plant pathologist unless content to remain a mere cata- 

 loguer is perforce compelled to become a specialist either on some 

 limited choice of parasites or on the diseases of a limited number of 

 hosts; thus we have the Fusarium specialist, the rust specialist, even 

 the Puccinia graminis specialist on the one hand, and the specialist on 

 fruit diseases, on cotton diseases, on potato diseases, etc., on the other. 

 Both kinds are necessary. The life histories of some fungus parasites 



