May 15. 191S 



Potato-Scab Organism 



131 



in a few tubes at the end of one month. A few of the remaining cultures 

 grew when taken to the laboratory, but the rest were dead. 



Exposure to cold weather, several degrees below zero centigrade, does 

 not always kill the parasite. During February and March, 1913, many 

 test-tube cultures were exposed immediately after inoculation to freezing 

 at outdoor temperatures and then again taken to the laboratory. The 

 exposure in no case was longer than one week. In no instance were the 

 organisms killed in tubes containing cooked potato cylinders, but in 

 some cases with beef-broth cultures an exposure of five days was fatal 

 when on some nights the thermometer registered as low as — 29° C. 



RAPIDITY AND VIGOR OF GROWTH 



Temperatures between 35° and 40° C. are most conducive to rapid 

 germination. They are decidedly less favorable for the further develop- 

 ment of the organism, except that at 35° the growth for the first day was 

 more rapid than at any other temperature tested. No colonies visible 

 to the unaided eye appear in cultures at 39.5°, and growth at this tem- 

 perature practically ceases within one week. On the other hand, growth 

 is very much retarded and slow below 20° C. Table I shows the com- 

 parative rates of germination and growth in cultures at various tempera- 

 tures and at different interv^als within one week. 



Table I. 



-Comparative rates of germination and growth of the common potato-scab 

 organism, at various temperatures and at different intervals 



