Minnesota Plant Diseases. 



569 



celium is at work witliin the leaf. On the under surface these 

 patches show at first a faint light grey shimmer, which later de- 

 velops into the grey mold-like growth of the fully developed 

 patches of the summer spores. These patches spread rapidly and 

 the whole leaf, in the course of a few weeks, dies, becomes brittle 

 and useless as a starch-making organ and dangerous as a pro- 



FlG. 197. — Downy mildew of grape. On the right is a healthy bunch of grapes; on the left 

 a bunch badly diseased. Original. 



ducer of the fungus spores. The so-called spores behave as do 

 those of the downy mildews generally, i. e., when they fall in very 

 moist surroundings they produce six or more swimming spores, 

 which swim about in the water drops and spread the infection on 

 the leaf. When they come to rest they germinate by sending out 

 infection tubes and the latter establish new regions of the myce- 

 lium. The leaf patches commence to appear in early summer. 



