Insect and Disease Control 273 



Course of development. 



The presence of this disease is indicated by the develop- 

 ment of warty excrescences on the larger roots and partic- 

 ularly at the crown of the tree just below the surface of the 

 ground. These excrescences are more or less spherical in 

 shape, and in size vary from bodies so small as to be un- 

 noticed to so large that they involve nearly the whole of 

 a main root at the point where a gall develops. 



The effect of crown-gall is in proportion to the extent 

 to which the roots are involved. If in a serious degree 

 the normal functions of the roots are interfered with and 

 the tree becomes weak and unthrifty, the foliage is light 

 colored — the tree looks "sick." These evidences become 

 more and more acute until the tree finally dies. 



Means of control of crown-gall. 



The only method of control is by preventive measures. 

 There is no cure, once a tree is infected. All nursery stock 

 should be examined very critically before it is planted in 

 order to detect every indication of small galls just beginning 

 to form. Every tree that show^s any evidence of them should 

 be burned. If a gall is discovered where it can be cut off, 

 its removal might delay somewhat the progress of the trouble, 

 but a tree so handled would remain under suspicion and 

 galls would be likely to develop subsequently. 



When crown-gall is discovered on a tree that has been 

 planted some years, the ultimate weakening therefrom may 

 be deferred somewhat by keeping the soil well enriched, 

 especially with nitrogenous plant-foods, thus providing as 

 favorable conditions as possible for tree growth. 



