﻿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 
  Spiroyyra. 
  

  

  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 
  Fl 
  <>- 
  186. 
  Grape 
  mildew 
  (Plasmopara) 
  

   properly 
  sprayed, 
  the 
  ill 
  ef 
  - 
  Fr 
  m 
  the 
  mycelium 
  within 
  the 
  cells 
  of 
  the 
  

  

  fects 
  may 
  be 
  reduced 
  or 
  

   prevented. 
  The 
  spray 
  kills 
  

   spores 
  which 
  are 
  upon 
  the 
  

   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 
  hyphre, 
  grow 
  between 
  

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

   grow 
  short 
  branches 
  (JiaustoricC), 
  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 
  

  

  grape 
  leaf, 
  haustoria 
  (B) 
  are 
  formed. 
  

   Upright 
  hyphse 
  (A) 
  hear 
  conidia. 
  These 
  

   conidia 
  divide, 
  as 
  at 
  D, 
  and 
  form 
  zoospores 
  

   (E). 
  Within 
  the 
  leaf, 
  oiispores 
  (C) 
  are 
  

   formed. 
  After 
  Duggar 
  

  

  