FUNGI AND FUNGOUS DISEASES OF PLANTS 239 



In addition to reproduction by asexual spores, sexual repro- 

 duction sometimes occurs. Tips of branches unite and form 

 heavy-walled zygospores, somewhat after the manner of zygo- 

 spore formation in Spirogyra. 



226. The grape downy mildew. It is not uncommon to see 

 the white patches of downy mildew upon the leaves of the 

 grape (fig. 185), and some- 

 times it appears upon the 



green shoots and the fruit. 

 In the central states, sup- 

 posed to be the original home 

 of this parasite, it has been 

 an injurious pest for many 

 years. In some localities it 

 well-nigh destroys the grape 

 crop at times, but when con- 

 ditions are thoroughly favor- 

 able to the vigorous growth 

 of grape plants, the mildew 

 does not seriously interfere 

 with the crop of grapes. 

 When plants which are at- 

 tacked by the parasite are FlG - 186 - Grape mildew (Plasmopara) 



properly Sprayed, the ill ef- From the mycelium within the cells of the 

 ,. n grape leaf, haustoria (B) are formed. 



fects may be reduced or Upright h yp h8e (A) bear conidia- ^ ese 



prevented. The Spray kills conidia divide, as at D, and form zoospores 

 i -i ,-, (E). Within the leaf, ob'spores (C) are 



spores which are upon the formed . A fter Duggar 



leaf's surface. 



227. Structure and nutrition of grape mildew. The surface 

 patches that are characteristic of downy mildew are not pro- 

 duced until some time after the leaves have had the parasite 

 growing within them. The threads, or hyphte, grow between 

 the cells of the leaf, and through the walls of these cells there 

 grow short branches (haustoria), which absorb food directly 

 from the cell contents of the host plant (fig. 186, J5). Thus 

 the parasite may grow by means of the food material made 



