No. 4.] PLANT DISEASES. 25 



ing to the practical man is but general and vague, — it stops 

 short of the most immediate and close applications. I would 

 maintain that for the best practical understanding of plant 

 diseases we must classify not according to cause primarily, 

 but according to plant ; that is, we must aim to understand 

 all the maladies of each crop, and that in their relation.';, 

 especially their relations to each other and to the develop- 

 ment of that particular plant. This will involve a consid- 

 eration of many things not ordinarily brought out in the 

 discussion of plant diseases. Among these are the origin 

 and history of that plant under human cultm'e ; the life liis- 

 tory of the individual plant, and a consideration of its devel- 

 opmental stages ; the relation and adaptation of the crop to 

 its environment, especiallj^ to clhuate and soil; the inter- 

 relations of the various maladies ; and, finally, the variations 

 in individual i)laiits and varieties as to their relative liability 

 to these maladies, — that is, their relative powers of disease 

 resistance. 



Every one of these matters may stand in the closest rela- 

 tion to the practical questions of the cause, spread and pre- 

 vention of plant diseases. That we may realize more fully 

 the importance of such considerations, let us again consider 

 and compare the apple and the potato as regards these mat- 

 ters. 



First, the apple. The origin of the apple as a cultivated 

 plant antedates recorded history. It is said that charred 

 remains of the fruit are found in the prehistoric lake dwellings 

 of Switzerland. For unnumbered thousands of years man 

 has been using, cultivating, selecting, seeding and perhaps 

 grafting this fruit. The apple is a native of southwestern 

 Asia and the adjacent regions of Europe, — regions lying 

 within the north-temperate zone, with seasonal changes and 

 temperature extremes essentially like our own, but charac- 

 terized by less uniform moisture. In cultivating this fruit 

 man has sought only its flesh. He has therefore confined 

 his attempts at modification by breeding and selection to the 

 increase of the amount of this, the amelioration of its flavor, 

 texture and keeping (|ualities, and incidentally to color char- 

 acters. As a result, we have to-day innumerable varieties 



