No. 4.] PLANT DISEASES. 27 



and mode of origin. As already noted, the climate of the 

 Asiatic and European regions where it is native is a dry one. 

 It is significant, upon reviewing the diseases of the apple, to 

 find that rarely if ever does serious disease result in our cli- 

 mate from insufficient water ; but, on the other hand, the 

 most serious fungous pests of New England, the scab and 

 the sooty blotch, and in the south the bitter rot, are associ- 

 ated with excessive moisture.^ 



It is further noteworthy that the parts where the most 

 rapid and serious injury is wrought by the widest variety of 

 maladies is the part most modified under human culture and 

 selection, — that is to say, the flesh of the fruit. This is the 

 weakened organ, attacked most violently by scab, and mold, 

 and blotch, and worm, and maggot, and curculio. But 

 recall how few of these pests really aftect the production 

 and maturing of an ample crop of seeds, which is, after all, 

 the essential end in the normal life processes of the apple as 

 a living organism. 



Finally, let us inquire as to the relation of varieties to dis- 

 ease. In nature there is always the struggle for existence, 

 and the inexorable law of the survival of the fittest has as 

 its corollary the elimination of the unfit. It is the operation 

 of this law through countless generations, we may believe, 

 which has given the vigor to the essential parts and organs 

 of the apple tree which enable each and all of them which 

 man has not disturbed to bear theu' quota of ailments with- 

 out real danger to life. 



But man, in his selection of varieties for pleasing flesh of 

 fruit, has scarce given practical consideration to this as yet. 

 Are there no differences, then ? Every boy knows there are. 

 He knows that in the old orchard the fruit of certain varie- 

 ties will ripen prematurely in advance of others ; and he 

 knows that it is because the worm, as well as he, has a 

 peculiar fondness for that fruit. Every one knows that the 

 scab ruins certain varieties, while others are practically im- 



' The author is indebted to Mr. M. B. Waite of the United States Department 

 of Agriculture for calling his attention to this relation, as well as to certain of the 

 other facts presented, in the course of lectures on apple diseases delivered by Mr. 

 Waite at the Graduate School of Agriculture, University of Illinois, July, 190G. 



