Chapter X _157_ Plant Diseases 



IS now 



Irish potato was at first believed to comprise a single species. It 

 generally recognized, however, that a number of species of actinomy- 

 cetes, all belonging to the genus Streptomyces, are capable of causing 

 potato scab. The most important of these species was first described 

 by Thaxter, in 1890, under the name of Oosfora scabies (415). This 

 name was later changed to Actinomyces chromogenus, then to Actino- 

 myces scabies (Thaxter) Giissow, and finally to Streptomyces scabies 

 (Thaxter) Waksman and Henrici. 



It may be of historical interest to cite the original description of the 

 potato scab organism by Thaxter: 



"Vegetative hyphae brownish, .06(0.6?)-1.0[x in diameter, curving 

 irregularly, septate or pseudoseptate, branching. Aerial hyphae at 

 first white, then gray, evanescent, breaking up into bacterial-like seg- 

 ments, after having produced single terminal spiral spores by the coil- 

 ing of their free extremities. Forming a firm lichenoid pellicle on nu- 

 trient jelly and usually producing a blackish-brown discoloration of the 

 substratum on which it grows, causing the disease known as scab on 

 potato tubers, and a similar disease of beet roots." This was as perfect 

 a description of a species of Streptomyces as could be given at that time. 



Since 1890, considerable information has accumulated concerning 

 the nature of the potato scab organism, its morphology, taxonomy, and 

 physiology. A number of problems are still, however, a matter of dis- 

 pute. This is true of the occurrence of the organism, the presence of 

 several causative agents, and their respective virulence. One of the 

 major reasons for the existing confusion has been the difficulty of pro- 

 ducing infection of potatoes or other susceptible plants under controlled 

 sterile conditions. 



Scab appears on potato tubers in the form of small brown spots. 

 These increase in size, with several infected spots commonly coalesc- 

 ing, and the underlying tissues becoming brown and pulpy. The in- 

 fected area is at first smooth and unbroken, but later the skin ruptures, 

 and a shallow depression is exposed. The base and edges of the scab 

 now become thickened with layers of cork laid down by the potato in 

 its attempt to cut off the disease from the underlying tissue. 



During the growth of the tuber, the stomata and the state of the 

 lenticels in the apical end offer means of entrance of infectious agent 

 into the tuber. When the lenticels first break open, the parenchyma 

 cells are exposed. This enables the organism to come into direct contact 

 with the cells. Fellows (121) emphasized that growth is essential 

 for infection. Large tubers exposed to the scab organism showed much 

 greater infection than small ones under the same conditions. The 

 pathogen affects the host by extending its influence through the mid- 

 dle lamella of the subepidermal cells. When the host is more mature, 

 it extends through the phellogen. The cells of the latter divide to 

 form new additions to the corky layer. S. scabies stimulates cell divi- 

 sion. Tubers pass through a period of susceptibility, followed by a 



