91 



it a knowledge of the life history and habits of the organism 

 ■which is the primary cause of that disease. 



The nature of nonparasitic or physiological diseases often is 

 obscure, but where the underlying cause can be determined 

 the remedy usually lies in changing or avoiding conditions 

 which produce it. 



In addition to the diseases which occur primarily in the 

 orchard, there are a number \^hich commonly affect the fruit 

 only in transit and storage. Some of the diseases which at- 

 tack fruit in the orchard continue to develop after it is har- 

 vested and placed in storage. 



The economic importance of controlling apple diseases is in- 

 dicated by the total loss due to diseases in the United States 

 for 1918 which was estimated at about 20,000,000 bushels. 

 In Massachusetts alone, the loss has been estimated at over 

 150,000 bushels. In 1920, in one small section of the State, 

 the value of the crop was reduced fully $200,000 by diseases. 



Descriptions of the more common diseases of Massachusetts 

 apples follow. 



Scab. 



This disease probably causes more loss to the apple crop of 

 the State than any other, though relatively few varieties are 

 seriously affected by it. Among the more susceptible of the 

 varieties commonly grown in Massachusetts are Mcintosh, 

 Fameuse (Snow), Fall Pippin and Rhode Island Greening. 

 Scab is not ordinarily a serious disease in Western Massachu- 

 setts, but in the eastern part of the State, especially on the 

 Mcintosh, it assumes great importance. 



Scab is caused by a fungus, Venturia incequalis, which at- 

 tacks young twigs, leaves, blossoms and fruit. Its occurrence 

 on twigs is rare. On the blossoms, it infects chiefly the pedi- 

 cels and calyx, and may sometimes cause severe blossom drop. 

 The disease is most in evidence on the leaves and fruit. The 

 first appearance on the leaves is in the spring. It then occurs 

 on the lower surface as a web-like growth, brownish or olive- 

 brown in color. Later, velvety spots of the same color form' 

 on the upper surface. Scab spots exhibit considerable vari- 

 ation in shape (Fig. 1), but the most common are approxi- 

 mately round, with a finely fringed margin. On both upper 



