Diseases of the Potato 201 



Powdery scab (Plate XI) 



Powdery scab is a disease of the tuber which is caused 

 by a slime-mold, Spongospora subterranea. It may be 

 easily observed at an early stage in young tubers as 

 brownish colored blisters. These, later, show as pits 

 filled v;,ith a brown powder, the spores of the parasite. 

 These pits are bordered by the torn skin of the tuber. 

 In storage the slime-mold may continue to be active, 

 causing a dry sunken rot about the pit. A cankerous 

 stage of the disease has been described as occurring in 

 low, wet lands, in which case the deeper tissues of the 

 tuber are invaded by the slime-mold and by other organ- 

 isms, so that it becomes badly rotted. The disease has 

 not been serious in the United States, and its develop- 

 ment seems to be arrested in the warmer potato sections. 



The parasite is disseminated by the distribution of 

 affected tubers and by any agent upon which the spores 

 may become lodged. The disease is diflScult to control, 

 but it is said to be materially reduced by seed treatment 

 and rotation of crops. Affected tubers should be de- 

 stroyed by burning or by boiling, and no tubers, even 

 though healthy, coming from fields known to be infested 

 should be used for planting. 



Tip-bum 



This is a disease chara,cterized by a browning and 

 shriveling of the tips and margins of the leaves. The 

 trouble may become quite severe and greatly reduce 

 the yield of tubers. It is often confused with blight, but 

 it is not due to parasites, but to hot, dry weather follow- 

 ing a period when the conditions of temperature and 



