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 Fic. 186. Grape mildew (Plasmopara) 
properly sprayed, the ill ef- From the mycelium within the cells of the 
fects 1 b d 1 grape leaf, haustoria (B) are formed. 
ects may be reducet OF Upright hyphae (.1) bear conidia. These 
prevented. The spray kills conidia divide, as at D, and form zodspores 
teat th (EZ). Within the leaf, odspores (() are 
spores which are upon eS formed. After 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 hyphe, 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, B). Thus 
the parasite may grow by means of the food material made 
