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 Beijerinckiw; (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 
