548 



SPECIAL PLANT PATHOLOGY 



killed presents a dark-brownish appearance. If the causative organ- 

 ism, which is a small rod-shaped bacillus, is sprayed upon the sweet 



pea plant, the disease makes its ap- 

 pearance from seven to ten days 

 after artificial infection and the 

 symptoms are similar to those pro- 

 duced in nature. The bacillus is 

 rarely found in chains and seldom 

 united in twos or fours. Its flagella 

 are not easily demonstrated, as they 

 are shed so readily that not more 

 than two to five may be stained 

 and these are generally quite short. 

 If properly fixed and stained, very 

 long delicate flagella may be dem- 

 onstrated, 8 to 12 in number, and 

 peritrichous. 



Sweet Potato (Ipomcea batatas), 

 Poir) 



Black-rot (Spheeronema fimbriata 

 (Ell. & Hals.), Sacc). — We owe our 

 past knowledge of this disease to 

 Halsted, who in 1890 described this, 

 as well, as other diseases of the sweet 

 potato. It is a seed-bed disease, a 

 field disease and a storage trouble. 

 It is characterized by irregular hard, 

 dark areas, or circular spots, varying 

 in size from that of a dime to that of 

 a silver dollar appearing on the skin 

 of sweet potatoes (Fig. 195). If the 

 root is injured, the fungus follows 

 the line of injury. The sprouts are 

 dwarfed and the foliage turns yel- 

 low. The end of the hank is black- 

 ened and charred and this is asso- 



FiG. 195. — Sweet-potato black 

 rot produced by a fungus, Sphar- 

 onema fimbriatum. (After Harler, 

 L. L.. V. S. Farmers' Bull. 714. 

 March 11, 1916.) 



ciated with a withering of the leaves which become black and crisp. 



