216 Diseases of Truck Crops 



manifested on the roots as a dry rot which appears 

 first as scattered sunken spots bordered by dark areas 

 (fig. 36 f). The pycnidia of the fungus are generally 

 absent from the spots, but they appear in great 

 abundance when the roots are placed under favor- 

 able conditions of moisture. 



Control. It is doubtful if Phoma rot can be con- 

 trolled by spraying the foliage in the field. Since 

 the disease is carried over in the roots, it would be 

 dangerous to feed them to stock or dump them on the 

 manure pile. Rotation should be practiced where 

 the disease has appeared more than once in the same 

 field. Care should be taken that no diseased roots 

 be permitted to enter the storage house or cellar. 

 The roots should be thoroughly dried before storing, 

 and the house or cellar should be kept moder- 

 ately cool and ventilation resorted to wherever 

 possible. 



POWDERY MILDEW 

 Caused by Erysiphe polygoni D. C. 



Powdery mildew has not been known to cause 

 any considerable damage to turnips in the United 

 States. It is characterized by the presence of 

 powdery white patches on both surfaces of the leaf. 

 Besides affecting the turnip, Erysiphe polygoni has 

 been recorded on about three hundred different hosts, 

 especially the garden pea. For methods of control, 

 see p. 367. 



