THE GRAPE-MILDEW. 



227 



to be due to Oidium TucJceri, the fungus which caused for- 

 merly a great deal of injury to the grape-crop in Southern 

 Europe, and especially in the Island of Madeira. The de- 

 velopment 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 Uncinula spiralis^ and belongs to a large 

 group of leaf-parasites, the Ferisporiacece. The dusty or 

 webby appearance of the leaves is caused by the growth of 

 the mycelium over the surface. The mycelial threads, al- 

 though they may cover a great part of the surface of the 

 leaves, do not enter into their interior, except that at inter- 

 vals the threads are furnished with little suckers, which just 

 penetrate into the external cells, and serve to attach the 

 mycelium. During the summer some of the threads grow 

 up from the surface of the leaf, and at the tip divide into a 



Fig. 6. 



Fig. 5. 



number of squarish ovoid cells (Fig. 4), which are spores cor- 

 responding to the conidial spores of the black knot. Later 

 in the season a number of round bodies (Fig. 5) are formed 

 on the threads. They are at first yellowish, and afterwards 

 black. These black bodies are hollow, and contain a num- 

 ber of sacs (Fig. 6), in which are spores which may be said 

 to correspond to the winter spores of the black knot. The 

 black bodies have attached to them a number of peculiar 

 threads or appendages, which are rolled up at the end, from 

 which the name Uncinula is derived. 



The second form of blight which occurs on grapes begins 

 to appear about the same time as the first, and may be mixed 

 with it on the leaf; or more frequently it occurs alone. 

 The first form of blight may be found on either side of the 



