300 MANUAL OF TREE DISEASES 



hemlock or fir has not been proved necessary for the appear- 

 ance of the rusts on poplars, although such an association 

 of the two required hosts is believed essential for the 

 existence of these rusts. If this is true, this rust can be 

 avoided by keeping larch, hemlock and fir separated from 

 poplars by a distance of several hundred feet. 



Powdery Mildew 



Caused by Uncinula solids (DC.) Winter 



The powdery mildew of poplar leaves is found throughout the 

 United States, but no great damage is done to the tree. It 

 occurs also throughout Europe and Asia and over its entire 

 range it affects the willows as well. The mycelium grows on 

 both sides of the leaves and forms either effused or definitely 

 circumscribed white powdery areas. The life histories and 

 control of the. powdery mildew fungi are similar for all species 

 and are discussed in general on page 37. There is apparently 

 only this one powdery mildew found on poplar. Some kinds 

 of trees are affected by two or more species. 



Yellow Leaf-Blister 



Caused by Taphrina aurea Fries 



The leaves of several species of poplar are affected by the 

 yellow leaf-blister. In Europe the black poplar is affected. 

 The size of the blisters may vary from very small to an inch in 

 diameter. They are yellow at a certain stage in their develop- 

 ment, due to the fruiting-structures of the causal fungus. Later 

 the color of the blisters changes from yellow to brown. The 

 mycelium of the fungus is confined to the space it makes for 

 itself between the cuticle and epidermal cells, usually of the lower 

 side of the leaf. No branches of the mycelium enter the leaf- 



