Waksman 



158 



Actinomycetes 



period of immunity. When the growing tubers are no longer forming 

 an epidermis with stomata or young lenticels, the period of susceptibil- 

 ity ceases. 



The anatomy of scab formation has been the subject of numerous 

 other investigations (219, 325). 



Fully developed scabs vary considerably in appearance, because of 

 different types of infection. In some cases, the depression formed in 

 the early stages of the disease is not raised to the surface by the sub- 

 sequent formation of cork, thus giving the affected potato a pitted ap- 

 pearance. In other cases, the scabs are raised by the formation of cork, 

 with the result that they stand out above the surface of the tuber in 



Table 42 : Certain characters of scab-producing actinomycetes (514) : — 



knob-like projections. These two forms of scab are often designated 

 as "pitted" and "raised" (298). 



Although numerous bacteria and fungi can be isolated from young 

 and old scabs on potatoes, only the actinomycetes— and these were iso- 

 lated only from young scabs— were found capable of causing infec- 

 tion (325). 



For many years, the idea of Thaxter prevailed that a single or- 

 ganism is concerned not only in potato scab, but in sugar beet scab as 

 well. Detailed studies of the organisms isolated from the various types 

 of scab eventually led, however, to the conclusion that more than one 

 species is involved in the causation of this disease. The multiple 

 origin of scab is now believed to be definitely established. It was even 

 suggested (272) that many actinomycetes found in the soil have patho- 



