Insect and Disease Control 277 



glossy, transparent, and hard. Later, in the presence of 

 much rain, the gum masses may swell and become sticky 

 and gelatinous in appearance. Hesler and Whetzel ^ enumer- 

 ate nearly twenty primary causes of gumming or gummosis, 

 including several fungous diseases, one or more bacterial 

 diseases, injuries due to insects and other mechanical means, 

 physiological troubles resulting from unfavorable soil or 

 climatic conditions, and others. The phenomenon of gum- 

 ming is, therefore, an expression usually of some disorder 

 rather than being itself a primary cause. However, wounds 

 do not always exude gum and it is believed by some au- 

 thorities that there is an exciting or stimulating influence 

 in some cases apart from the wound itself that induces the 

 exudation of gum. An enzyme is commonly held by such 

 authorities to be the inciting cause of the exudate. 



Methods of control. 



It is obvious when a phenomenon may result from so large 

 a number of causes that the first step in control is to deter- 

 mine the causal agent of the condition and then remove 

 the cause or condition or apply such remedy as may be sug- 

 gested by circumstances. 



Little-leaf, or California yellows 



The cause of this trouble appears to be a physiological 

 disturbance due to unfavorable soil conditions, particularly 

 with regard to moisture. It has no relation to "peach- 

 yellows" of the East. 



1 " Manual of Fruit Diseases." 



