Diseases of the Sweet Potato 163 



ing a large quantity of organic matter, such as manure. 

 Such soils should be avoided. It is likewise more se- 

 rious during a wet season and on low wet ground. The 

 treatment of the soil with fungicides or fertilizers has 

 not been effective as a means of control. 



LEAF-BLioHT (Plate VIII) 

 (Phyllosticta batatas) 



Leaf-blight appears on the upper side of the leaves, 

 either as roundish or angular spots. These vary in size 

 from /4 to % inch in diameter and are defined by a 

 distinct line around them. Within the spot are several 

 blotches or dark patches which are caused by the spores 

 of the disease. So far it is not known that this disease 

 is parasitic on any other plant and only on the leaves of 

 potatoes. It is thought to live over the winter on the 

 dead leaves. The disease usually manifests itself on 

 the mature leaves. 



Leaf -blight occurs principally in the southern states, 

 rarely appearing on the east coast north of Virginia, 

 or in Iowa, Illinois, and Kansas. So far the disease has 

 not been of enough consequence to justify treatment. 



LEAF-SPOT (Plate VIII) 



(Septoria bataticola) 



This fungus is similar in general appearance to leaf- 

 blight. It manifests itself in the form of spots % to % 

 inch in diameter ; these spots are scattered promiscuously 

 over the foliage. They are white surrounded by a 

 brown border. In the center of these spots are dark 

 areas very similar to the ones described in leaf-blight. 

 These are the clusters of spores before they are released 



