268 BACTERIAL DISEASES OF PLANTS 



enters must be soaked in 1 :1000 mercuric chlorid water for 40 

 minutes. 



In wet autumns, in moist soil, there is often a wholesale 

 rotting of potato tubers due to the entrance of the parasite 

 through the lenticels and it is a very instructive experiment to 

 demonstrate lenticellate infection in the laboratory. Sorauer 

 was, perhaps, the first person to see bacteria enter the potato 

 tuber in this manner. The writer saw it many years ago 

 (1886) and has obtained on potato tubers in recent years very 

 typical and beautiful infections by way of the lenticels (Fig. 202), 

 using Bacillus melanogenes (which as received by me was a mix- 

 ture of Bacillus phytophthorus and Bacillus solanisaprus). 

 For this purpose one should select smooth, sound, recently 

 harvested tubers, wash clean and plunge for 30 hours under 

 distilled or autoclaved or even non-sterile tap water, to which a 

 young agar-streak culture of this organism has been added. 

 If the variety is susceptible, numerous infections centering in 

 lenticels should appear within a few days, and soon the interior 

 of the tubers should be wholly disintegrated. The sterility of 

 the water used is of no great consequence so long as it does 

 not contain parasites, and so long as the surface of the tuber 

 itself is not sterile, the aim being to set up conditions like those 

 obtaining in ordinary infections in wet earth. Of course, checks 

 to which the parasite has not been added should be held and 

 these will remain sound unless their surface happens to be 

 contaminated with this or some similar parasite, but the tubers 

 must not be asphyxiated. 



What proportion of the wholesale rot of potato tubers in the 

 soil in wet autumns is to be ascribed to Bacillus phytophthorus, 

 Bacillus solanisaprus, and similar bacterial parasites, and what to 

 mere saprophytes following asphyxiation cannot be determined 

 until a great many more studies have been made of the flora of 

 rotting potatoes. 



Determine 



FOR THE ORGANISM: 1. Morphology. Size in microns, 

 form, aggregation of elements (Do chains or filaments occur? 

 See No. VI), motility on margin of a hanging drop (How long 



