Diseases of the Potato 199 



planted on land where crops have been rotated. Lime, 

 wood ashes, manure and other materials giving alkaline 

 reaction should not ordinarily be used on potato soil in 

 connection with the growing of that crop. If it is neces- 

 sary to apply any of these substances in order to grow 

 other crops the application should be made in the fall 

 after digging the potatoes. Manure may be applied best 

 as a top dressing to the meadow. 



Black-leg 



This disease, which is caused by the bacterium Bacillus 

 phytophorus, affects the vines and tubers. Infected 

 vines have a lighter green color than is usual and have a 

 stiff upright growth instead of the normal spread of foli- 

 age. The leaflets have a tendency to roll about the mid- 

 rib. The plant also may be smaller than it would natu- 

 rally be, though this is not always so. The base of the 

 stem is quite black and somewhat shriveled. The 

 diseased area commonly extends two or three inches 

 above the ground, sometimes much farther. It also 

 extends downward to the old seed tuber and may extend 

 into the stolons and young tubers. 



The affected tubers may show a stem-end black-rot, 

 and when cut lengthwise sometimes show a black, foul- 

 smelling center decay and even a hollowed condition. 

 This center decay may be present with little evidence of 

 it at the surface. 



The disease is distributed mainly by affected seed 

 tubers, or by bruised, cracked or decayed tubers harbor- 

 ing the organism. Control consists in the selection of 

 seed tubers from healthy plants, in discarding bruised, 

 cracked and rotted tubers, and in seed treatment. 



