MICHIGAN STATE POMOLOGICAL SOCIETY. 471 



from a diseased tree to healthy trees in the neighborhood, and 

 thus capable of indefinite extension. A modification of the 

 contagious theory is, that the disease is imparted to healthy 

 trees from diseased ones by means of the pollen carried by bees 

 and other insects from the flowers of diseased to the flowers of 

 healthy trees. A sufficient answer to this theory is the fact 

 that trees often exhibit the Yellows before they have ever 

 flowered. 



CAUSE OF THE YELLOWS. 



A still greater diversity of opinions exist as to the cause of 

 the Yellows. 



Some assign as the cause, a condition of enfeebled vitality, 

 arising from poor cultivation, want of pruning, over-bearing, 

 growing on poor soil, and this soil still further impoverished 

 by cropping, etc. That enfeebled vitality alone is not a suffi- 

 cient cause is shown by the fact that other fruit trees are sub- 

 jected to the same agencies, and yet do not have the Yellows. 

 The apple, pear, and plum are often enfeebled and impover- 

 ished in the same way ; the pear may have the blight, and the 

 plum the black knot, but none of these exhibit the true 

 Yellows. 



Others speak of this disease as a '•consumption," but the 

 assigning of human diseases as explanation of vegetable dis- 

 ease is founded upon a false analogy. If we wished to be face- 

 tious, we might reply that one characteristic symptom of con- 

 sumption is a cough ; and I think no one ever heard a Yellowed 

 peach tree cough ! 



Others give the peach borer as the cause of Yellows ; and 

 undoubtedly many cases of supposed Yellows have arisen from 

 the injuries inflicted by the borer. 



Others have suggested that "the disease arose from a deficiency 

 of mineral matter to form the prussic acid of the peach-pit ;'* 

 but the prussic acid consists of carbon, hydrogen, and nitrogen, 

 and contains no mineral matter whatever. 



Finally, a fungus attacking the root or bark of the tree has 

 lately been brought forward as the true cause of the Yellows. 



