Vegetable and Field Crops 219 



On the surface of tlie tul)er slightly depressed, dark- 

 colored areas appear wliile 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 conditions of heat 

 and moisture. A daily mean of 22° to 23° C. (72°-74° F.) 

 accompanied by moist weather conduces to its spread. A 

 daily mean temperature above 25° C. (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. Young plants 

 grown fi-om infected seed remain dwarfed, assume a reddish 

 brown color, and when proper weather conditions obtain, 

 furnish the spores to start the general attack. 



Marked difference in resistance exists among varieties. 

 Thus in Maine 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 vari- 

 eties the following are reputed to possess some degree of 

 resistance: Carmen No. 3, Sir Walter Raleigh, Rural New 

 Yorker No. 2, Rural Blush, Green Mountain, State of Maine, 

 Delaware, Enormous, and White Beauty. 



Field trials have shown that certain German and English 

 varieties are more highly resistant than the American vari- 

 eties. It is a hopeful sign, therefore, that American potato 

 breeders ai-e now giving attention to disease resistance and 

 that new varieties of greater excellence in this respect may 

 be expected in the future. 



Healthy potatoes only should be used for seed. Diseased 

 potatoes may be recognized by their reddish brown surface 

 color, which extends toward the center of the potato; also 

 by the slight shrinking of the suiface in such parts, or by the 

 spotting described above. 



The crop should be sprayetl as recommended under the 



