30 PHYCOMYCETES 



Atkinson. Damping off. Bull. Cornell Univ. Agric. Exper. 

 Sta. 94 : 233-272. PI. 1-6. 1895. 



Order 5. PERONOSPORALES. 



The Peronosporales are parasitic plants preying mostly on the 

 higher plants and form an extensive and destructive group of 

 fungi. There are two types representing two families. One type 

 commonly known as white rust, forms milk-white glistening clus- 

 ters of conidia under the epidermis of shepherd's purse and other 

 crucifers, and the other forms whitish mould-like masses of branch- 

 ing conidiophores on the under surface of leaves of various plants. 

 The latter type includes two of the most destructive diseases of cul- 

 tivated plants affecting the grape and the Irish potato respectively, 

 and commonly known as downy mildews. The species infesting 

 the potato has been known to destroy from one-third to one-half 

 of the entire crop of potatoes of a whole state ; and the one affect- 

 ing the grape after its introduction into France in 1878 caused 

 consternation among the vineyard owners of southern Europe 

 until methods of treatment were found for the prevention of its 

 ravages. In this country it caused the abandonment of numerous 

 vineyards during the same period. 



The downy mildew of the grape attacks all green portions of 

 the vine, its mycelium growing as an internal parasite and draw- 

 ing its nourishment by means of haustoria (PL j, f. j>). The 

 symptoms are : 



1. Yellowish spots on the upper surface of the leaf with mould- 

 like conidiophores on the corresponding lower surface, the spots 

 ultimately becoming brown and dead. In bad cases the entire 

 leaf may become involved in which case it soon shrivels as if 

 burned. 



2. The berries if attacked early do not attain one-fourth their 

 normal size and turn brown or gray if conidia are formed. If 

 attacked later the full-grown berries will show the same colors. 

 This is sometimes known as the brown rot, but must be clearly 

 distinguished from the black rot, which is due to an entirely 

 different parasite. 



The downy mildew of the grape, in common with other species 

 of the order, produces two kinds of spores which are different in 

 origin, form and function. 



