GRAPE DISEASES 241 
six or eight extremely small naked spores. Each of these 
has two cilia which give it the power of motion; on account 
of this motile character these smiall naked bodies have been 
named swarmspores. The production of swarmspores, which 
really initiate the disease, begins in February and continues 
until June. They are formed and discharged within 24 hours. 
The swarmspores or their parent spores, the conidia, are spat- 
tered in drops of water or mud by heavy rains to the leaves of 
the vine. After reaching the leaf, the swarmspore, which is still 
a naked protoplasmic mass, soon comes to rest and surrounds 
itself with a thin wall. Then a germtuhe is developed which 
penetrates the leaf by way of a stoma, always on the lower side 
of the leaf. If aswarmspore is on the upper surface of the leaf, 
it swims over the edge of the same to the lower side. The 
germtube within the tissues develops into mycelium which 
brings about the various changes in the affected parts as evi- 
denced by the symptoms already described. Within a short 
time the fungus masses itself beneath a stoma, through which 
opening several (four to ten) conidiophores pass. These grow 
from most: of the stomata in the infected region, each conidio- 
phore reaching a height of about one-fiftieth of an inch. In 
mass they present a whitish, downy appearance — the downy- 
mildew. These structures form branches, on the ultimate 
tips of which are borne conidia. The conidia are blown to 
other susceptible parts of neighboring vines, where new spots 
are formed. In the autumn, affected leaves fall to the ground, 
carrying the fungus within them. Before winter arrives 
oospores, as many as 200 to the square centimeter, are formed. 
In the spring the oospores are set free by a rotting of the leaves, 
germination occurring in the soil. 
A temperature of 77° to 82.4° Fahr. is best for the growth 
of the fungus. Shaded, moist situations are favorable for its 
development. Long, dry periods do not kill the pathogene, 
although such conditions seriously check it. Heavy rains 
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