304 MANUAL OF FRUIT DISEASES 



occurs more or less all over the world. Gumming is not con- 

 fined to stone-fruits, but is known also on citrus trees. Gum- 

 ming of the former only is considered here. 



Symptoms. 



Gummosis, or gumming, is usually evident in the spring, 

 particularly after cold rains and when growth is active. Masses 

 of gum, at first glassy or transparent and soft, then amber- 

 colored and hard, exude from the twigs, branches and trunks. 

 Examination will usually show the presence of a break in the 

 bark through which the gum exuded. The gum becomes 

 swollen and sticky in the presence of water. The casual 

 observer will note gum only on the external portions; how- 

 ever, it is formed internally. Sometimes gum is developed in- 

 ternally and there is no evidence of it externally. Gumming 

 often accompanies the blighting of blossoms or twigs, canker- 

 wounds on limbs and trunks and wounds left by pruning 

 operations. 



Came. 



The phenomenon of gum-flow, gummosis, is associated with 

 a variety of conditions, and therefore it is difficult on finding 

 gum-exudation to attribute it to any definite cause. Wounds 

 are made inadvertently, and many factors are capable of making 

 them ; and wherever a stone-fruit tree is wounded, gum is very 

 certain to exude. But wounds do not alw r ays exude gum ; an 

 exciting factor is often essential after the wound is made. Just 

 why a tree should exude gum is a matter which has been dis- 

 cussed for years. It is now quite generally held that it is due 

 to some enzyme produced by the host-protoplasm. Some 

 authorities, however, believe that gummosis cannot be due to an 

 enzyme. Others simply say that the gum exudes in response 

 to some stimulus in an effort on the part of the tree to protect 

 a wounded surface, and that the phenomenon represents a 

 retrogressive change in the cell contents. The real causal 

 factor, whatever it may be, passes up the ducts, enters adjoin- 



