THE POWDERY MILDEW 



31 



together ; and the mildew often covers the whole surface 

 of the leaf, either as a thin coating through which the 

 green surface can be seen, or as a thick, felt-like cover- 

 ing, which entirely conceals the leaf surface. This mil- 

 dew is composed of a great number of minute white 

 threads, branching in all directions, which form the 

 mycelium of the powdery mildew fungus. In the fungi 

 of this group the mycelium is external, i. e., instead of 

 developing on the inside of the leaf of the host-plant, it 

 develops on the outside, and in order to get nourishment 



c d 



FIG. 15. APPLE POWDERY MILDEW. 



c, Mycelium formingsummerspores; c/, winter spore case (perithecium). Magnified. 

 \ 



the mycelium threads send little suckers into the cells 

 of the leaf and absorb their contents, for the benefit of 

 the fungus. Soon after the mycelium gets well devel- 

 oped it assumes a powdery appearance, due to the pro- 

 duction of great numbers of the minute white spores, 

 called the summer spores, or conidia (Fig. 15 V). These 

 are very light, and are scattered by every breath of air. 

 When one of them falls upon an unaffected leaf, where 

 sufficient moisture is present, it germinates and starts 

 the disease anew. By means of these summer spores a 



