410 PLANT DISEASES 



already in the soil, but the treatment may be used when planting 

 is to be on a field free from the disease. Practically all of the 

 crop will then be free from scab. 



It has been observed that when the spores are in the soil the 

 crop is often made more ' 'scabby " by the use of fresh horse manure, 

 applied to the soil the same year the crop is grown. When potatoes 

 are grown in rotation over a series of years the lime and manure 

 may be applied to the field for some other crop, such as corn or 

 clover. , 



Potato Blight. Irish potatoes are affected with two distinct 

 blights. One is called early blight and the other late blight. 



FIG. 286. Potato tubers badly attacked by the scab disease. (U. S. D. A.) 



Early blight is much less injurious to the crop, as it seldom 

 affects the crop of early potatoes much before they are ready for 

 harvest. Sometimes it attacks the leaves and gradually covers 

 the whole plant in the form of dark spots surrounded by concentric 

 rings of different shades on the leaves. The entire crop may be 

 destroyed by it. 



Late blight attacks the plants near the ground and the spread, 

 both upward and downward, is quite rapid. The leaves and 

 stems turn dark brown as they wilt. The spore-distributing stage 

 of the disease is seen in the darkest areas. Spores often fall to the 



