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 small 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 germtube is developed which 

 penetrates the leaf by way of a stoma, always on the lower side 

 of the leaf. If a swarmspore 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 



