252 HORTICULTURE FOR SCHOOLS 



operator in a sort of knapsack, which connects with a bellows. 

 The operator works the bellows as he walks along the row, 

 giving each vine a thorough dusting. A power-sprayer for 

 the purpose is now coming into use. Two applications, some- 

 times three, are necessary to control the disease. The fungi 

 grow most readily when warmth and moisture are present. 

 Clear cool weather acts as a check and reduces the number 

 of applications of sulfur necessary. 



Mildew affects many other plants beside the grape. It is 

 found frequently on rose bushes, and appears also to some 

 extent on peaches, apples, currants, garden beans, and others. 

 In addition to the measures named above, plants should 

 usually be placed in positions where they re- 

 ceive fairly constant sunlight, for this assists 

 materially in keeping down the disease. 



363. Crown-gall (Fig. 128) is a very com- 

 mon disease of deciduous trees. As the name 

 indicates, it affects the crown of the root, just 

 below the surface of the ground. It is a bac- 

 terial disease. When the bacteria gain access to 

 the root of the plant, they cause enlargement 

 of the diseased tissue, producing a gall-like ap- 

 pearance. This gall may first be located on one 

 side of the root only, but as the disease pro- 

 gresses it gradually spreads around the root, 

 until the entire crown is affected. As soon 

 Fl ^ 1 1 1 28 -~ Crown " as this occurs, a circle of the cambium layer 

 is killed, connection with the root is severed, 

 and the tree dies. From the time the disease first appears, 

 several years usually pass before it kills the tree. 



No cure is known. When the infestation is light, it is some- 

 times recommended to cut away the diseased portion, and 

 paint with some antiseptic to stop the spread of the disease. 

 In general, however, this is not practicable. It is always 

 necessary, when buying young fruit-trees, to watch the roots 



