19231 



JENNISON — POTATO BLACKLEG 13 



phytes quickly follow Bacillus atrosepticus in the tissues of both 

 stem and tuber, it is often difficult to obtain the pathogen. By 

 using extra precautions in carrying out the ordinary isolation 

 methods one may obtain pure cultures of the causal organisms. 

 A positive identification of the organism may then be made by 

 careful comparison with the description of B. atrosepticus on p. 



43. 



ECONOMIC ASPECTS 



Smith ('20) has come to regard the disease under consideration 

 "as one of the most serious diseases of the potato." Earlier 

 writers on the subject have unanimously emphasized its eco- 



nomic 



Many cases are reported in the 



where upwards of 50 per cent of the plants in a field were de- 

 stroyed by this disease. However, it appears that a loss of 5 

 per cent of the plants in a potato-growing district represents the 

 usual amount of damage done by blackleg. In districts where 

 the disease has been long known and where control measures 

 have been intelligently applied, the losses have been greatly re- 

 duced, the average for the United States as a whole being about 

 0.5 per cent annually. 



Dispersal and infection.— Probably the blackleg patho 

 most usually disseminated by the more 



of 



fected seed stocks. Morse (16) mentions a striking illustra- 

 tion of this in a field in Idaho where he found blackleg which 

 "undoubtedly came all the way from Scotland ... in five 

 years." 



Without doubt the etiological agent is carried by wind and wa- 

 ter, especially the latter, in the irrigated fields of the West. As 

 pointed out by Smith ('20), invasion of the tubers through wa- 

 ter-gorged lenticels can take place, but whether it does or not 

 under natural conditions in the field is not known. The writer 

 has planted healthy seed pieces in heavily infected soil but the 

 disease did not develop in the plants thus grown. Similar re- 

 sults were recently attained by Shapovalov and Edson ('21). It 

 appears to the writer that it still remains to be proven that the 

 blackleg parasite can gain entrance to the vines or tubers through 

 an unbroken epidermis. 



The larvae of insects have been found by the writer working 

 in and on the affected tissues, but there was no positive evidence 

 that they wei e a tive agents in the dispersal of the disease. Von 



