Vegetable and Field Crops 225 



introduced through foreign shipment in 1911. It is now 

 prevalent in certain parts of Canada and in Maine, Massachu- 

 setts, Nebraska, New York, Washington, Oregon, and 

 Minnesota. The tubers are infected when very young. At 

 maturity the infected area becomes a pit, filled with dusty, 

 yellow spore-balls; the pit bordered l)y remnants of the 

 tuber's skin. The sale value of the potato is decreased, the 

 yield lessened, and its value for seed purposes lost. 



Precautions should be taken to guard against infected 

 seed. 



Black-scurf '-"•'• ■-''^' "'^ {Corticium vagum B. & C, Rhizoc- 

 tonia). — Though particularly prevalent in the Middle West 

 and Rocky Mountain states, this pest is also known generally 

 in the East, from Connecticut southward. Infected tubers 

 are largely responsible for the contamination of new soil. 

 They may be recognized by the superficial, irregularly 

 shaped, scurfy, dark-brown patches (sclerotia) of fungous 

 threads varying in size from a pinhead to a grain of wheat. 

 They are seen clearly only when the tuber is wet. 



Soon after planting the young sprouts are attacked by 

 this waiting fungus. Black spots appear near the surface of 

 the ground, destroying the bark and often girdling the stem. 

 In such cases aerial tubers often form, and the plants are 

 malformed and stunted. A dark network of fungous threads 

 is sometimes seen upon the subterranean parts. The fungus 

 advances above ground and develops a gray spore-bearing 

 layer about the green stem, often extending upward several 

 centimeters, causing the young shoots to wilt, droop, and die. 

 Often a loss of one-fourth to one-third of the plants follows. 

 Stems that survive this early attack, but succumb to later 

 attack, produce only a few underground tubers, small and 

 unsalable. 



Stem lesions similar to those of black-scurf are also pro- 

 duced by a number of different fungi. 



Only clean seed should be used. If there is any doubt, it 

 should be treated as is recomnu^nded for potato common- 

 scab, but even this treatment is not effective if large sclerotia 



