PERONOSPORALES 



217 



described as having sporangia and sporangiophores like those of 

 Plasmopara but with germination always by germ tube. 



As the method of germination throughout the family is some- 

 what inconstant and to a degree dependent on the conditions of 

 the emdronment it seems better to place the emphasis in generic 

 separation on morphology rather than on differences in germina- 

 tion. On this basis the genera Rhysotheca and Pseudoplasmopara 

 are here merged with Plasmopara. 



Fig. 78.- — Plasmopara australis Swingle. Sporangiophore 

 (After Schwarze 1917.) 



and sporangia. 



Approximately fifteen species of Plasmopara are known for 

 North America. Of these P. viticola (Berk. & Curtis) Berl. & 

 De Toni., cause of downy mildew of cultivated and wild grapes, is 

 best known. This species was first reported from the United 

 States by Schweinitz in 1834. It apparently originated in this 

 country on the wild species of Vitis. It was well described by 

 Farlow in 1876. Two years later it was discovered in the vine- 

 yards of southern France and very quickly became a menace to 

 the grape industry. Its rapid spread over Europe and Asia 



