THE GRAPE DOWNY MILDEW (PLASMOPARA VITICOLA) 355 



fertilization forms an oospore which can germinate. This 

 peculiar feature, called parthenogenesis, meaning reproduction 

 by an egg without fertilization, has been mentioned before 

 (page 50). 



The aquatic habit, reproduction by zoospores, and character 

 of sex organs support the theory that the Water Molds are 

 degenerate forms with the Green Algae as their ancestors. The 



Fig. 305. — Leaf of the Grape, showing the downy areas caused by the 

 Downy Mildew, Plasmopara Viticola. After W. H. Hein. 



coenocytic character of their hyphae suggests a close relationship 

 with the Siphonales. 



Downy Mildews (Peronosporales). — These Fungi, which are 

 parasites on the higher plants, cause some serious plant diseases 

 of which the Grape Downy Mildew and Potato Blight are notable 

 ones, and will serve to illustrate the habits of the group. There 

 are about 100 species known and the order is so named because 

 of the downy patches which they produce on the diseased portions 



of the host. 



The Grape Downy Mildew (Plasmopara Viticola). — i he 

 Downy Mildew of the Grape, shown in Figure 305, is a very 



