;6o 



Bulletin 163. 



less definite bands, often just at the surface of the ground. The 

 diseased bands, or areas, may, however, appear much lower, as 

 in figure 62. It is very probable that this is determined by soil 

 and moisture conditions. The scabby protuberances are abnor- 

 mal developments of corky tissue stimulated to excessive growth 

 by the presence of the fungus. Professor Arthur has noted 

 sunken scabby spots on the surface of the beet, and he explains 

 these as early injuries which failed to develop further, when the 



63. — Scabby potatoes. 



conditions were probably unfavorable, and future growth of the 

 beet has left them rather as pits than as excrescences. 



b. The Cause of the Disease and Its Prevejition. 



In 1890 Professor Thaxter discovered that potato scab is caused 

 by the growth on the surface of the tuber of a fungus which he 

 named Oospora scabies. When scabby potatoes, such as are 

 represented in figure 63, are placed in a moist chamber, a light. 



