VEGETABLE AND FIELD CROPS 265 



discernible at considerable distance from the field. The 

 disease usually soon shows on the stems also, which in turn 

 blacken and die. 



On the tuber the surface shows slightly depressed dark- 

 colored areas, while internally the normal white color 

 changes to dirty brown. If the soil is dry, dry rot results ; 

 if wet, the tubers decay as wet, slimy, ill-smelling masses. 

 In storage the disease continues, and the loss sustained in 

 the field may be greatly augmented. 



This blight is closely dependent upon weather conditions 

 of heat and moisture. A daily mean of 72°-74° F. accom- 

 panied by moist weather is best for its spread. A daily 

 mean temperature above 77° F. for a few days retards its 

 development. 



Although the conspicuous signs above noted first attract 

 the attention of the general observer, the disease really 

 commences as soon as the sprouts appear upon some of 

 the young plants grown from infected seed. Such shoots 

 remain dwarfed, assume a reddish brown color, and when 

 proper weather conditions obtain, furnish the spores to 

 start the general epidemic. 



Marked difference in resistance exists between different 

 varieties. Thus Woods of Maine found that the Rust- 

 proof showed the highest resistance, about 1 per cent of 

 disease, while no other varieties tried gave less than 30 per 

 cent of disease. Of the standard crop varieties the follow- 

 ing are reputed to possess some degree of resistance : Car- 

 men No. 3, Sir Walter Raleigh, Rural New Yorker No. 2, 

 Rural Blush, Green Mountain, State of Maine, Delaware, 

 Enormous, and White Beauty.^ 



' Jones, L. R., U.S. Dept. Agr. Bur. Plant Indus. Bui. 87. 



