Orchard Pests and their Control 359 



DISEASES AND INSECTS OF THE APPLE 



Fungous Diseases 



Blight (Bacillus amylovorus). See under Pear. 



Crown -gall. The subject of crown-gall is of vital impor- 

 tance in the semi-arid country, since under our conditions the 

 organism that is responsible for this disease of fruit trees and 

 plants finds congenial surroundings for growth and distribution. 



The so-called galls are irregular outgrowth of tissue that com- 

 monly form around the crown of a tree just below the surface of 

 the ground. They also occur frequently on the roots, but are 

 very different in appearance from the swellings that are produced 

 by the attacks of woolly aphis, which, unfortunately, are also 

 very destructive. 



There are two forms of galls, known as the soft and the hard. 

 They are much alike in external appearance, but differ in struc- 

 ture. " Hairy-root " is possibly a distinct disease, but its effects 

 are much the same. It receives its name from the quantity of 

 fine roots which may or may not arise from a gall. All of these 

 forms appear to be equally destructive. 



The galls increase rapidly in size, when the conditions are 

 favorable, and so interfere with the processes of nutrition that 

 the vigor of the tree is greatly impaired. In many instances the 

 death of the tree is but a matter of a few years. The point of 

 attack being underground, the infected trees are commonly un- 

 noticed until they begin to fail. This stage may be recognized 

 by the weak growth and yellow appearance of the foliage. 



This disease first began to attract the attention of experiment 

 station workers in 1892, when the California station published a 

 bulletin on the subject. This was followed by a number of articles 

 from different stations, and in 1900 Tourney, then of the Arizona 

 Experiment Station, published a bulletin in which he attempted 

 to prove that crown-gall is induced by attacks of a slime mold. 

 His results, however, have never been verified, and the exact 

 cause of the disease is not yet definitely known. 



In the spring of 1907 Smith and Townsend, of the Depart- 



