222 FARM FRIENDS AND FARM FOES 



the mycelium in the diseased potatoes. When they are 

 planted, a new crop of spores apparently is produced, 

 either on the seed potato itself or upon the stalks and 

 leaves that grow from it. Many of the downy mildews 

 have special winter spores by means of which the fungus 

 lives from one season to another, but these seem not to 

 have yet been certainly found in this species affecting the 

 potato. 



The best methods of controlling Potato Blight are the 

 planting of tubers from fields in which the disease was not 

 present, spraying with Bordeaux mixture, rotation of the 

 crop, and the selection of resistant varieties. 



When a parasitic fungus attacks one plant, it is very 

 likely to attack other closely related plants. The tomato 

 is closely related to the potato. Consequently it does not 

 seem strange that there is a Tomato Blight caused by 

 the same fungus that causes the Potato Blight. It is 

 often destructive in wet summers in the Eastern states and 

 during damp winters in southern California. Rotation of 

 crops and spraying with Bordeaux mixture are effective 

 preventive measures. 



OTHER DOWNY MILDEWS 



A parasite closely related to the Downy Mildew of 

 potatoes is the Downy Mildew of Lima Beans. This at 

 times is very destructive in many parts of the United 

 States, sometimes greatly reducing or almost destroying 

 the crop. It appears as a thick white covering upon the 

 pods, injuring the tissues of the wall and often extending 

 the damage to the young beans inside. It spreads chiefly 

 by means of summer spores, and apparently passes the 

 winter in the seed in the form both of dormant mycelium 

 and of distinct winter spores. 



