170 Diseases of Truck Crops 



in the development of soil stain. Sweet potatoes, 

 grown on very light sandy soils, especially those 

 which are hilly, are usually free from the disease. 

 Heavier lands, or those rich in humus, rarely produce 

 a clean crop. The application of manure favors the 

 spread of the fungus and increases the stain. In 

 fact, the manure itself is often a carrier of the disease, 

 since diseased roots of all sorts find their way 

 ultimately to the manure pile. The trouble is also 

 carried directly with the seed stock. This, when 

 planted in the seed bed in diseased condition, will 

 produce 100 per cent, of diseased sprouts. Wet 

 weather is favorable to the spread and increase of 

 stain. During wet seasons the disease is more 

 plentiful than in dry seasons. 



The Organism. The spores are born in distinct 

 chains which break up very readily when moistened 

 (fig. 28 a-1). The conidiophores are born on the 

 surface of the epidermis (fig. 28 n). Careful ob- 

 servation of these chains have shown them to be 

 made up of from 10 to 28 conidia. The spores (con- 

 idia) are one celled, hyaline, with a greenish tinge, 

 but never dark or brown. The spores readily germi- 

 nate in water or in any nutrient medium (fig. 28 o). 



VINE WILT OR YELLOWS (STEM ROT) 



Caused by Fusarium batatatis Woll. Fusarium 

 hyperoxysporum Woll. 



The terms "stem wilt," "vine wilt," or "yellows" 

 are commonly used to describe this disease, and 



