94 Journal of Agricultural Research voi. viii, no. 3 
In 1913 some of the crop raised from these selected and disinfected seed 
tubers were sent to Director T. C. Johnson, of the Virginia Truck Experi¬ 
ment Station, for planting at Norfolk. Out of over 3,000 plants pro¬ 
duced from this seed only two doubtful cases of blackleg were reported. 
At the same time two lots of seed tubers purchased in the open market 
gave 6.5 and 7.9 per cent, respectively, of diseased plants, and another 
lot sent in for testing gave 16.8 per cent. 
Undoubtedly much can be gained by uprooting and destroying all 
blackleg plants in the field as soon as seen, taking care to remove also all 
tubers which have formed; but the writer has no experimental data 
bearing on this point. It should be recommended not as a substitute for 
seed selection and disinfection,* but rather to supplement them, for the 
latter are, of course, equally important in the control of several other 
diseases which are carried by the tubers. 
COMPARATIVE STUDIES OF THE CAUSAE ORGANISMS 
Of the various organisms which have been isolated and described in 
different parts.of Europe and in Canada as being the cause of Schwarz - 
beinigkeit , or blackleg of the stem of the potato, and the attendant decay 
of the tubers, the following are available for study: Bacillus atrosepticus 
Van Hall, B. phytophthorus Appel, B. solanisaprus Harrison, and B. 
melanogenes Pethybridge and Murphy. As far as could be learned from 
reading the published articles and descriptions, the different investi¬ 
gators who named the species in question made no comparative, cultural 
studies in the laboratory, under identical conditions, of the previously 
described species and the organisms which they themselves had isolated. 
They based their conclusions with regard to the nonidentity of the species 
upon the manner in which they found their own organisms to differ from 
the descriptions previously published. 
Therefore, when the writer began his studies of this disease in Maine 
in 1907, several interesting questions presented themselves. From both 
a scientific and a practical point of view the most important question was, 
Is the disease in the United States, especially in Maine, caused by one or 
more of the named species of blackleg-producing bacteria or by one 
differing from each of them in certain well-defined characters ? In other 
words, if the previously described organisms were collected together and 
subjected to the various differential tests usually employed in describing 
a species of bacteria under exactly identical laboratory conditions, would 
these differences still hold or would they all prove to be identical with 
each other and with those found associated with the disease in Maine ? 
The exerience of the writer (16) in a similar study of the very closely 
related group of bacteria causing a softrot of various vegetables sug¬ 
gested that the last question possibly might be answered in the affirma¬ 
tive. Accordingly, after considerable preliminary study had been made 
of some 18 different strains of organisms isolated in different parts of 
