Diseases of the Potato 191 



is not affected to a greater depth than one-fourth to one- 

 half an inch, ahhough such affected tubers in a heavy 

 wet soil or even in storage may be invaded by saprophytic 

 fungi or bacteria that produce a soft, malodorous condi- 

 tion known as wet rot. 



The fungus causing late blight lives as a mycelium in or 

 near the margin of the diseased area in affected tubers. 

 When such tubers are planted under conditions favorable 

 to the fungus, namely, a warm moist soil, the mycelium 

 will grow into shoots often weakening them and some- 

 times destroying them. 



When these affected sprouts succeed in growing, they 

 appear above the ground as a slender, weak stalk often 

 hidden by healthy neighboring ones from the same tuber. 

 On these weak stalks the fungus, during moist weather, 

 forms its fruiting bodies on which are produced a large 

 number of spores. These are blown by air currents to 

 neighboring healthy vines, where, if the vines are wet, 

 they germinate in two to four hours in an interesting 

 manner. An opening appears at the tip of the spore, 

 and six or more zoospores are pushed out as naked masses 

 of protoplasm that are able to move about in the water 

 on the leaf surface by means of whip-like strands of 

 protoplasm extending from their bodies. After moving 

 about for an hour or so, they settle down on to the sur- 

 face of the leaf and after a time send out a little tube, the 

 germ-tube. The germ-tube is able, after a few hours, to 

 penetrate the surface of the leaf. Once inside, the germ- 

 tube grows rapidly into a mycelium extending between 

 the cells and even penetrating them. These cells be- 

 come disorganized and turn brown and from them the 

 mycelium obtains its nourishment. Cell after cell is 

 killed in this manner, and the discoloration resulting upon 



