318 



Wisconsin State Horticultural Society. 



as well on wild grapes as on cultivated, and neither species is 

 yet known to occur in Europe, although both are common 

 through the Eastern United States. 



You may have noticed that the leaves of many cvltivated 

 grapes are apt to look dusty after the first of August. The dust- 

 iness, if such it really were, would of course disappear after a 

 heavy rain. But such is not the case. During the damp weather 

 the dusty look increases, and after a while the leaf dries and 

 shrivels. As the leaf shrivels the dustiness disappears, and in its 

 place we see a number of very small black bodies scattered all 

 over both surfaces of the leaves. In some cases, instead of look- 

 ing dusty, the leaves seem to be covered with a tolerably thick 

 white web, which extends to the leaf stalks, and in extreme cases 

 to the grapes themselves. The blight is often supposed to be due 

 to Oidium Tuclceri, the fungus which caused formerly a great deal 

 of injury to the grape crop in Southern Europe, and especially in 

 the island of Madeira. The development of that fungus is only 

 partly known, and there is no proof that our fungus is the same. 

 The American fungus referred to is called TJncinula spiralis, and 

 belongs to a large group of leaf parasites, the Perisporiacrc. The 

 dusty or webby appearance of the leaves is caused by the growth 

 of the mycelium over the surface. The mycelial threads, although 

 they may cover a great part of the surface of the leaves, do not 

 enter into their interior, except that at intervals the threads are 

 furnished with little suckers, which just penetrate into the exter- 

 nal cells, and serve to attach the mycelium. During the summer 

 some of these threads grow up from the surface of the leaf, and 

 at the tip divide into a number of squarish ovoid cells (Fig. 15) 



Fig. 15. 

 which are spores corresponding 

 to the conidial spores of the 

 black knot. Liter in the sea- 

 son a number of round bodies 



Fig. 10. 



