320 Wisconsin State Horticultural Society. 



of branching threads, on whose tops are borne oval bodies (seen 

 in the branches in figure eighteen), which are the conidial spores. 

 The threads do not, however, grow all over the surface of the 

 leaf, but make their way from the interior into the air, through 

 the holes which abound on the under surface of most leaves, and 

 are known by the name of "breathing spores." If we follow the 

 threads still farther, we shall find that they penetrate through all 

 parts of the leaves and stems, making their way between the leaf 

 cells. The threads are also furnished with small suckers, which 

 push their way directly through the walls of the plant cells into 

 their interior. The bodies which we may call the winter spores 

 of this blight are not found on the surface, but are imbedded in 

 the leaf. They are round and have thick walls. The name of 

 the second form of grape blight is Peronospora vilicola. 



A comparative study of the two blights we have just described 

 is instructive. They both first appear on the leaves about the 

 first of August, and both cause them to shrivel and drop off. 

 One sometimes attacks the grapes, while the other does not. The 

 two blights may be distinguished with the naked eye by an ordi- 

 nary observer, as one 'forms a sort of dusty-looking web on any 

 part ; the other, frost-like spots on the under surface of the leaves. 

 Both have conidial spores, which grow on stalks in the air. Both 

 have what we may for convenience call winter spores, which 

 ripen late in the autumn. Those of the Uncinula are in the round 

 black bodies on the surface of the leaf; those of the Peronospora 

 are in the interior of the leaf. 



A microscopic examination shows us that it is not correct to 

 speak of grape milldew, or blight, as a distinct disease. We 

 have just seen that there are at least two different fungi which 

 produce a blight ; and the two differ decidedly in their habits 

 and growth, so much so, that the means taken to prevent the 

 growth of one will not apply to the other. Let us consider this 

 practical point more at length. We will suppose that the grape 

 grower recognizes that his plants are attacked by the first form of 

 blight described — Uncinula spiralis. As a microscopic exami- 

 nation shows that the fungus is on the surface, and not in the 

 interior of the leave?, it is plain that the object should be to 



