314 Diseases of Truck Crops 



Powdery Scab 

 Caused by Spongospora subterranea (Wdll.) Johns. 



Powdery scab may justly be considered a danger- 

 ous disease. The trouble has undoubtedly been of 

 European origin. In the United States the disease 

 is now found in Presque Isle, Me., Chateaugay, N. Y., 

 Nehalem, Ore., Hastings, Pla., Inohomish, Wash., 

 and Virginia, Minn. The trouble has been carefully 

 investigated by Melhus and Rosenbaum.' 



Symptoms. Powdery scab attacks the young root- 

 lets, forming galls resembling in size those of legume 

 nodules (fig. 59 c, d). At this stage infection does not 

 take place on the tubers. In fact it is not unusual 

 to find the total root system affected with galls, while 

 the tubers remain free. Thus if we look for the dis- 

 ease in the field a search should be made for infection 

 on the roots and rootlets. 



Infection on the tubers is evidenced at first by 

 minute discolored areas on the epidermis. Six to 

 eight days later, the spots increase in size, become 

 raised and somewhat jeUylike. Powdery scab on the 

 tubers cannot easily be mistaken for common scab, 

 Actinomyces chromogenus. In powdery scab the 

 sori are more often circular and not as extended as in 

 common scab. In powdery scab, the border of the 

 pustules is virtually raised, forming a cupUke sorus 

 or pit (fig. 59 a), and the pits are deeper and at matu- 



' Melhus, I. E., and Rosenbanm, J., U. S. Dept. of Agr. Jour. Agr. 

 Research, 7 : 213-354, 191 1. 



