10 THE ROYAL SOCIETY OF CANADA 



that the commoner fruit tree diseases of the east are rare in California, 

 that on the contrary certain soil diseases, crown gall, and gummosis 

 are frequent there; that the late blight of potatoes is especially pre- 

 valent in Nova Scotia, but gradually becomes less frequent to the 

 south; that cabbage yellows develops very rapidly in dry, hot weather 

 following transplanting, while the black rot of cabbage flourishes in 

 damp, cool weather; that the sooty mold of tomatoes is frequently 

 abundant on outdoor tomatoes in the southern States, while it is 

 found in greenhouses only in the north; that the stinking smut of 

 wheat is propagated by soil borne spores west of the Rocky Mountains, 

 but only by seed borne spores east of the Rockies; that clover dodder 

 has repeatedly found its way in foreign seed into Ontario, has grown 

 for one season and then died out. In short, environmental factors 

 probably play a very important part, if not a prédominent one, in 

 nearly all parasitic diseases. The interesting part about it is that in 

 some diseases the greater effect appears to be on the susceptibilities of 

 the host, in others they act more strongly on the parasite — its capaci- 

 ties for germination, infection and growth. The fact of the matter is 

 that there is a complexity of relations peculiar to each disease, and 

 that up to the present in few instances only have these relations been 

 analyzed separately and in detail. Yet the solution of these problems 

 would often be of the greatest practical value, directly influencing 

 prophylaxis. The lack of knowledge of this type is strikingly shown 

 in the inability of pathologists to forecast with reasonable certainty 

 the probable course of introduced fungi. Take for example the blister 

 rust of white pine, which is highly destructive in parts of Europe. In 

 spite of the fact that it has been known now for a generation it is still 

 impossible to say whether or not the conditions in the white pine areas 

 of North America are favorable to its becoming a devastating plague. 

 We must wait for an answer now by actual testing out in experiment 

 plots, meanwhile permitting it to roam at large to do its worst; there 

 are obvious disabilities attendant on such a means of discovery. 



So far I have been referring to environmental factors with refer- 

 ence to growing crops, but they also continue to operate on their 

 products up to the time of their actual utilization by the consumer 

 and throughout this latter period they should be largely controllable. 

 It has been stated by Adams in his book on Marketing Perishable 

 Farm Products that "at least 25 per cent of the perishables sent to 

 wholesale markets is hauled to the dump pile" ; and another authority 

 is responsible for the statement before the International Apple Shippers 

 Association that "frequently one-half or more of a perishable crop is 

 lost by careless handling." A large part of the losses that accrue 

 between the grower and the consumer result from decay due to molds 



