136 INTERNATIONAL CONGRESS OF VITICULTURE 



Ravaz and Verge (1913) also described an exactly similar method fully cor- 

 roborating these observations. 



Oospore germination may begin as early as the latter part of February, 

 though at this time it is not as abundant as later in the spring, March till 

 June. 



The oospores will retain their vitality for at least a year. Leaves con- 

 taining oospores were collected in October, 1912, allowed to remain on the 

 ground during the winter of 1912-13 and then kept in the laboratory in a 

 thoroughly dried condition until March, 1914. At this time they were 

 moistened and after being maintained about three weeks in these conditions 

 germination occurred in a small percentage of the oospores. 



Freezing seems to be necessary, since germination has only been obtained 

 from those oospores which have been allowed to remain on the ground 

 until at least January or February. 



The conditions essential for germination are very similar to those 

 necessary for conidial germination. A temperature of about 23 C. and an 

 abundant supply of moisture seems to be optimum. The leaves are allowed 

 to rot for a week or more in a warm moist place and thereafter it requires 

 usually about five to eight days in order to obtain germination. 



The time necessary for the complete formation of the primary conidium 

 and the emission of the zoospores is about 24 hours. The germination of 

 two oospores was followed from the time that the conidia were 8^ and 11/i 

 respectively in diameter at 12:35. The promycelium was still completely 

 filled with protoplasm. At 4.45 both conidia were full size, being 27.3/x and 

 39/i respectively in diameter and the septum separating them from the 

 promycelium was formed. The next morning they were in a similar con- 

 dition but at 12 m. germination, by means of zoospores, occurred. 



Numerous motile bodies have been observed in cultures which apparently 

 contained no germinating oospores and it was at first thought that possibly 

 germination might occur by zoospores directly but when these motile bodies 

 were fixed in osmic acid and stained with methylene blue it could be seen 

 readily that they possessed but one fiagellum. Furthermore, their globose 

 shape and size (10 to 13/0 preclude the possibility of their being zoospores 

 of Plasmopara viticola. Melhus (1914) has observed a parasite of oospores 

 which produces similar zoospores, hence it may be that we are also dealing 

 here with such a parasite. 



With these points in mind it appears certain that early spring infec- 

 tions arise from the oospores. Though the process has not been followed, 

 certain observations have indicated that this is the case. 



The leaves lying on the ground about the vines during the winter, are 

 in an advanced stage of decomposition by spring, thus only a short period 

 of warm, rainy weather is necessary to complete this process. The driving 

 rains, also occurring at this time, aid in breaking up the thoroughly rotted 

 leaves, thus more or less completely exposing the oospores and facilitating 

 conidial formation and dissemination. 



The conidia may germinate in the soil water or may themselves be 

 spattered to the lower leaves of the vine during heavy rains. Arbois de 

 Jubainville (1883) has stated that he has seen mildewed spots on the leaves 

 opposite bits of mud. The writer has also observed the mildew in the season 

 on the mud-bespattered leaves. This would indicate that some spores, either 



