292 



FUNGOUS DISEASES OF PLANTS 



of the tuber, often making its appearance when the tuber is very 

 young, and sometimes not until it has reached a considerable size. 

 This discoloration very commonly, though not invariably, has its 

 origin in one of the roughened points, or lenticels, which are scat- 

 tered over the surface of the potato, and after it has once appeared 

 may extend quite rapidly to the adjacent tissue, becoming deeper 



in color and being associated 

 with an abnormal corky de- 

 velopment of the parts in- 

 volved, which often cover a 

 considerable area. This area 

 may constitute a more or less 

 irregular scab-like crust over 

 the surface, or more fre- 

 quently may become deeply 

 cracked and furrowed, the 

 depth and extent of the injury 

 depending in a great meas- 

 ure upon the stage at which 

 the tuber first became dis- 

 eased ; those which are at- 

 tacked while very young 

 showing, as might be ex- 

 pected, by far the most deep 

 seated injury" (Fig. 127). 



If scabby potatoes are 

 carefully harvested and im- 

 mediately examined, there 

 will be found associated with 

 the disease an evanescent 

 grayish film. This film is made up of extremely delicate, minute, 

 refractive, branched filaments, which break up into bactericidal 

 cells. Some branches are curved, and spore-like structures are 

 also produced within certain cells. 



Experiments demonstrate that the fungus may persist in the soil 

 several years. A few scabby potatoes are sufficient to spread the 

 organism to a bin of clean tubers. To secure potatoes free of scab, 

 clean tubers should be planted in soil free from the fungus. 



FIG. 128. A SUGAR BEET AFFECTED WITH 

 SCAB 



