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Causation of Plant Diseases 



"\'ory l'(>\v forms anionji; the nctinomycetes 

 arc capable of causiuii; plaiil diseases. In spite 

 of tlie great ahuiKlance of aetinomycctes in 

 nature, especially in the soil, the numl)er of 

 plants attacked by them, as compared to 

 the number of plants attacked b}^ bacteria, 

 fungi, and viruses, is rather limited. Except 

 for two species — the Irish potato and the 

 sugar beet — plants subject to infection by 

 actinomycetes do not occupy a \'ery promi- 

 nent place in human economy. 



Potato Scab 



An extensi^•e amount of literature has ac- 

 cumulated dealing with the causation of po- 

 tato scab, the development of the disease, the 

 organisms invoh'ed, the effect of environ- 

 ment, and methods of control. Our main 

 concern here is the causative agents of the 

 disease. 



Organisms 



The organisms capable of causing com- 

 mon scab of potatoes were at first believed 

 to comprise only a single actinomycete. Grad- 

 ually it came to be recognized that a number 

 of species, or at least a number of races or 

 strains, all belonging to the genus Strepto- 

 myces, are capable of causing the infection. 



In 1890, Thaxter first described the organ- 

 ism causing potato scab under the name 

 Oospora scabies. This was later changed to 

 Actinomyces chromogenns, then to Actino- 

 myces scabies (Giissow), and finally to Strep- 

 tomyces scabies (Waksman and Henrici). 



Because of its historical significance, the 

 description given by Thaxter is (juoted here : 



"Vegetative hyphae brownish, .06 [0.6?] to 1.0 /* 

 ill diameter, curving irregularly, septate or pseu- 

 (loseptate, branching. Aerial hj'phae at first white, 

 then gray, evanescent, breaking up into bacteria- 

 like segments, after having produced single 

 terminal spiral spores by the coiling of their free 

 extremities. Forming a firm lichenoid pellicle on 

 nutrient jellj^ 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." 



The above description fits in perfectly 

 well with that of a typical species of Strepto- 

 myces. 



Thaxter's belief that a single organism is 

 concerned with potato scab, as well as with 

 sugar beet scab, prevailed for many years. 

 More recent studies of cultures of actinomy- 

 cetes isolated from \'arious types of scab 

 suggested the probability that several species 

 are in\'olved in the formation of scab. Most 

 investigators now tend to believe in the 

 multiple origin of this disease. It has even 

 been suggested that many actinomycetes 

 foiuid in the soil ha\'e pathogenic tendencies 

 which they may lose, if the host is not pres- 

 ent. If a suitable host is provided, the patho- 

 genic habits are reaccjuired. On the other 

 hand, some investigators still believe that 

 not only is the organism causing potato scab 

 a single species, but that S. scabies can cause 

 root necrosis in seedlings of wheat, pea, soy- 

 bean, radish, and a variety of other plants 

 of man}^ families. 



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