FUNGI IMPERFECTI 291 



The disease is most common throughout the United States, and 

 doubtless throughout the potato-producing regions of Europe as 

 well. It is not positively demonstrated, however, that all of the 

 surface injuries known as scab are properly referable to the fungus 

 here discussed as the causal organism, yet it is highly probable that 

 potato scab as a common disease is generally due to Oospora scabies. 

 Sturgis and others have found turnips (Bras sic a campestris], 

 beets, and mangels (Beta vulgaris] susceptible to this disease. Car- 

 rots (Daucus Carotd] and parsnips (Pastinaca sativa) are not re- 

 garded as susceptible. It is possible, moreover, that this fungus 



FIG. 127. POTATO SCAB 



may occur upon the less conspicuous roots of some other plants, 

 but it is typically a disease of fleshy roots. 



Before the scab organism had been isolated and careful inocu- 

 lation experiments made, a great variety of causes were assigned 

 by observers and investigators, various bacteria and fungi, also 

 insects and myriapods being held responsible for these injuries. 



The result of Thaxter's studies in 1 890 furnished proof that the 

 common form of scab in New England is caused by a minute par- 

 asitic fungus tentatively designated as above. The disease 1 " first 

 shows itself as a minute reddish or brownish spot on the surface 



l Thaxter, /. c., 1890. 



