Minnesota Plant Diseases. 



335 



crease in size and number with great rapidity until the whole 

 leaf finally becomes yellow, then dries up and shrivels. The 

 oldest leaves, i. e., those nearest the hill, are usually first affect- 

 ed and then the disease travels toward the tip of the host plant 

 with great speed. The spread of the disease is particularly fa- 

 vored by hot weather and by a damp atmosphere. The yield of 

 cucumbers is quickly affected, because the whole plant is rapidly 

 weakened or may be entirely destroyed. On the older plants 



FIG. 108. Downy mildew of muskmelon. Blighted vine i 



one finds on close examination a fine down, sometimes of a pur- 

 plish tinge, covering the under side of the leaf. This is caused 

 by the fungus threads, which bear the summer spores. The 

 latter are produced in a similar manner to those of the downy 

 mildew of grapes and are carried by the wind to other leaves 

 and plants. Here they form a large number of swimming 

 spores, which further scatter the infection under proper condi- 

 toins of moisture. The swimming spores germinate into infec- 

 tion tubes, and thus establish a mycelium within the leaf. This 

 fungus has not been known to produce winter spores, prob- 

 ably does, however, under rare conditions. The method of 

 wintering over is therefore at present unknown. It undoubt- 

 edly does, nevertheless, pass the winter safely, as has been 

 shown by the experience of cucumber growers in many places. 



