PLANTS FROM AN ECONOMIC STANDPOINT 345 



proper conditions of heat and moisture are present, they 

 produce swarm spores that soon germinate, each one 

 sending out a slender tube which enters a near-by breath- 

 ing pore of the leaf. Once inside the leaf they grow 

 rapidly and send out numerous branches which absorb 

 the contents of the cells and even pass down through the 

 stalk to the tubers below. Wherever they go they kill the 

 plant cells quickly, 

 and the term blight 

 is quite applicable 

 (Figure 306). In 

 a few days the 

 plants of the whole 

 field may present 

 a dry, parched ap- 

 pearance, since the 

 roots (mycelium) 

 of the mildew in 

 the leaves send out 

 branches which de- 

 velop millions of 

 spores. These may 

 be carried by the 

 slightest wind to other parts of the field. This fungus 

 commonly passes the winter in diseased potatoes ; hence 

 great care should be taken to secure good seed potatoes. 

 Spraying with Bordeaux mixture will prevent the spread 

 of the disease. 



Other mildews attack lima beans, onions, citrous fruits, 

 grapes, and many other fruits and vegetables. The brown 

 rot of the grape and of the lemon are downy mildews 

 which are at times quite destructive. 



Potato Scab. This fungous disease attacks potatoes 



U. S. Dept. of Agriculture. 



FIG. 306. Advanced Stage of Early Blight on 

 Potatoes. 



