CHAPTER XXIII. 



FUNGI (Concluded): THE WILLOW MILDEW. 

 (Uncinula salicis.) 



224. Description of the mildew. — The willow mildew belongs 

 to a very interesting group of the fungi known as the powdery 

 mildews. These mildews are very common on the leaves, and 

 even stems, flowers, and fruits, of various plants. It is a very 

 easy matter to find them during the summer or late autumn and 

 to press a number of the leaves to preserve for future study. 



The mycelium grows on the outside of the parts of the host, 

 so that it gives a whitish, "mildewed" appearance to the 

 affected places. Very short branches (haustoria) from the 

 mycelium enter the epidermal cells of the host and draw nutri- 

 ment from ihe leaves or other parts, and supply the fungus with 

 the materials for growth. This nutriment is taken at the 

 expense of the host, and often considerable injury to it is thus 

 done, which results in a sickly appearance of the host, or even 

 in a deformity, the leaves or stems being curled or dwarfed. 

 Immense numbers of small, colorless spores (gonidia) are borne 

 in chains on some of the threads, and these piled up on the 

 surface of the leaf give it a powdered appearance. 



After this powdery stage of the fungus has formed, 

 another kind of fruit of the fungus is developed. This may be 

 detected by numerous minute black specks seated on the white 

 mycelium, as shown in fig. 114. Each one of these black 

 specks is a fruit body. 



134 



