33 2 



Minnesota Plant Diseases. 



the border of the spot. Here are produced the spores, which 

 are formed in a manner peculiar to the potato blight and its 

 close relatives. The method of spore formation serves to dis- 

 tinguish these forms from the downy mildew of vines and 

 other downy mildews. The spore-producing threads pinch off 

 spores from their apices and then the thread grows past the 

 spore, shoving the latter to one side. It grows on for a short 

 distance and then produces another spore apically. The 

 threads show somewhat pointed ends. They are, moreover, 



FIG. 166. Potato blight. Early stages of the blight on the leaves. After Clinton. 



usually much branched, so that a miniature bush-like structure 

 is produced and each branch terminates in a spore. These 

 spores, as is true for most of the downy mildews, are in reality 

 spore cases, for when placed in water they later give rise to a 

 large number of swimming spores. When the latter come to 

 rest they germinate into a tube which causes infection of the 

 host plant. As far as is known at present, no winter spores 

 are produced. The mycelium, however, is capable of living in 

 the above ground stems and in the tubers of the potato, and 

 may live in the latter over winter, producing a brown rot of 

 the tubers. In the following spring they can again cause in- 

 fection by growing up into the stem and leaves. It is there- 



