POWDERY SCAB OF POTATOES. 97 



ance when washed. A Httle later the whole tuber may become 

 shrunken and shriveled, but this condition the writer has not 

 seen under the cool storage conditions of this State. In later 

 stages also, fine black dots may appear on the surface of the 

 tuber representing sclerotia of the fungus. These are quite 

 readily made out by means of a hand lens. 



While the above description applies very well to the mo;^t 

 common tn^e of the disease, the writer recently collected sev- 

 eral tubers showing spots undoubtedly caused by Spondyloc- 

 ladium which, when compared with powdery scab and common 

 scab, proved to be confusing to several individuals who were 

 quite familiar with both of these diseases. The affected spots 

 usually varied from one-eighth to one-fourth inch in diameter, 

 they were approximately circular in outline, and usually either 

 level with the surface of the tuber or slightly depressed. On 

 the same tuber all stages could be found from the small discol- 

 ored areas on the skin which are typical of the earlier stages oi 

 silver scurf to the spo'ts described as resembling scab. These 

 scab-like spots were brownish in color, the central portion being 

 frequently, although not invariably, darker than the periphery. 

 The spots are also frequently surrounded by a somewhat 

 lighter brown ring. This seems to be made up of densely inter- 

 woven, light brown threads of the fungus. These latter also 

 are usually found entirely covering the whole spot. If the spots 

 are large and somewhat irregular they may, on superficial ex- 

 amination, be classed as common scab. If they are small, unless 

 one has given the matter considerable attention he is apt to call 

 them spots of powdery scab the surfaces of which have been 

 removed by friction in rubbing against other potatoes. Figure 

 49 represents a potato affected by this type of the disease. 



Rhizoctonia Scab. The fungus Hypochnus solani P. & B. 

 ■commonly known as Rhizoctonia from its non-fruiting stage 

 has been attributed in some instances as the cause of a type of 

 ■scab. In inspecting potatoes for powdery^ scab tubers showing 

 k-sions apparently produced by this fungus are sometimes found. 

 These sipots, particularly in advanced stages, are more of a pit 

 than a scab and should not be confused with powdery scab, al- 

 though potatoes have been sent to this Station on the supposi- 

 tion that they represented this last type of disease. 



