Jan. 15,1917 
Blackleg Disease of Potato 
117 
columns of this table are given the number of hours which elapsed from 
the time a 10-c. c. broth culture was inoculated with a 2-mm. loop of a 
24-hour-old broth culture, before the first visible traces of clouding of 
the medium could be observed, observations being made hourly. In all 
cases the tubes of media were placed in the incubator at the desired tem¬ 
peratures for a few hours before inoculation, and care was taken not to 
remove them therefrom any longer time than was necessary to make 
the inoculations and observations. 
Table III .—Effect of temperature on rate of growth of the organisms 
Organism. 
First evidence of clouding of broth cultures at— 
5° C. 
io° C. 
15° C. 
20° C. 
25 ° c. 
30° C. 
35 ° C. 
B. atrosepticus . 
B . solanisaprus . 
B. melanogenes . 
IIIA. 
SE. 
IIP. 
B . phytophthorus from Appel. 
B , phytophthorus from Schus¬ 
ter. 
Hours . 
• 84 
96 
5 2 
84 
96 
192 
192 
192 
Hours. 
27 
27 
27 
28 
27 
59 
64 
60 
Hours. 
22 
'22 
22 
21 
21 
27 
30 
22 
Hours. 
*3 
*3 
*3 
14 
16 
16 
18 
13 
Hours. 
II 
II 
II 
II 
II 
*3 
13 
8 
Hours. 
12 
12 
12 
12 
12 
l6 
12 
II 
Hours. 
No growth. 
Do. 
Do. 
Do. 
Do. 
Do. 
16. 
12. 
It will be seen that while the optimum for the two nonpathogenic 
strains, carried under the name of B. phytophthorus is about the same 
temperature as for the six pathogens, the former grew at 35 0 C., while 
the latter did not. 
Both agar slants and beef-broth cultures were used to determine the 
maximum temperature for growth in the case of B . atrosepticus , B. 
solanisaprus , B. melanogenes , and the three pathogenic strains isolated in 
Maine. On beef-extract broth neutral to phenolphthalein this was not 
far from 33 0 C.; if anything, slightly below this. None of the six pro¬ 
duced visible clouding of broth at 34 0 C. and above. On agar slants the 
growth was scanty or absent at 32 0 and 32.5 0 , B. atrosepticus , SB, and 
IIP giving the most frequent failures to produce growth on agar at these 
temperatures. All of the pathogenic strains would show very evident 
growth in broth at 32 0 in 18 hours, and also clouding in the same period 
of time, with less regularity and with some failures to produce clouding 
at all at 32.5 0 . Undoubtedly the somewhat lower maximum tempera¬ 
ture for growth on agar was due to the rapid drying out of the surface of 
the medium. The results given above were checked a number of times, 
since they were unexpected on account of Harrison having reported 
growth with B. solanisaprus at 37°- 
The thermal death point for B . atrosepticus , B. solanisaprus , B. melano¬ 
genes and the three pathogenic strains isolated from Maine was found 
to be approximately 46.5° C. Retests at this temperature did not 
