184 MANUAL OF FRUIT DISEASES 



due to one or more of several factors. These factors act in a 

 stimulative manner. Among such causal agents may be noted : 

 (1) mechanical and insect injuries; (2) unfavorable soil and 

 climatic conditions ; (3) fungi ; (4) bacteria. 



Mechanical injuries such as those produced by bruising, or by 

 cultivating tools, may induce gum-flow. Pruning wounds often 

 exude gum. Insects, such as borers, produce injuries through 

 which gum commonly exudes. 



The relation of soil and climatic conditions to the formation 

 and flow of gum is not well understood. Trees in low places 

 where the soil is excessively wet are often more subject to 

 gummosis than others. On the other hand, trees on well- 

 drained soil may exude gum. Not infrequently frost induces 

 gummosis. 



Fungi are common causal agents in gum-flow. Some of the 

 more important ones will be noted : (1) the blight fungus, 

 Coryneum Beijerinckii; (2) the die-back pathogene, Valsa 

 leucostoma; (3) the black-knot fungus, Plowrightia morbosa; 

 (4) the brown-rot fungus, Sclerotinia cinerea; (5) the root-rot 

 fungus, Armillaria mellea. Doubtless many other fungi 

 induce gumming. 



It will be seen that gummosis is associated with a variety of 

 conditions of the plant, but it is often difficult or impossible 

 to attribute a given case of gum-flow to any one cause. Gum 

 is formed internally and may not always exude. It is formed 

 in pockets which are not visible on the surface of the bark. 

 Usually, however, the bark ruptures and the gum flows 

 out. 



With reference to bacteria as the cause of gummosis, the 

 development of Bacterium Cerasi will now be considered. 

 Many facts in the life-history of the organism are lacking. 

 The bacteria apparently lie dormant in the bark through the 

 late spring, summer, fall and early winter. In the case of limb 

 and trunk cankers the bacteria may or may not become active 



