304 Maine Agricultural Experiment Station. 1919. 



replanted with healthy tubers just a month after the second 

 planting. This time healthy plants were produced which ma- 

 tured without any signs of blackleg whatever. Exposure to 

 temperatures close to the freezing point for from i to n days, 

 retarded the activity of the blackleg organisms as the period of 

 exposure increased, but did not prevent their causing a complete 

 decay of the tubers after the latter were planted in pots in the 

 greenhouse, and, under the favorable conditions of moisture 

 and temperature in the pots of soil in the greenhouse, they 

 failed to retain their vitality for as long as 30 days. 



Duplicate samples of 3 different soil types were taken in 

 the fall. These were placed in small fruit jars, a virulent cul- 

 ture of B. atrosepticus was poured over each, and the covers of 

 the jars screwed on tightly. One set of jars representing each 

 soil type was buried 5 inches deep in soil outside while the other 

 set were stored in the basement of an unheated building. At- 

 tempts to isolate the blackleg bacteria from these different sam- 

 ples of the soil in the spring were unsuccessful. The experi- 

 ments mentioned above, taken together, indicate that the black- 

 leg organism remains alive in the soil but a very short time even 

 under favorable conditions. Also infected tubers left in the 

 ground at digging time decay to such an extent that they do not 

 germinate or if they do germinate the sprouts are immediately 

 killed by the parasite. This explains why volunteer plants are 

 always free from the disease, even though blackleg was preva- 

 lent on the same field the season before. 



In order to test the possibility of blackleg spreading from 

 plant to plant in the field, or from diseased tubers remaining in 

 the soil from the season before, and which might not entirely 

 decay during the previous fall and winter, quite an extensive 

 experiment was carried out during two successive seasons. 

 Pots of different soil types were sunk in the ground out of 

 doors, with their tops flush with the surrounding surface. 

 Healthy potato plants growing in these pots were watered at 

 intervals, beginning shortly after they came up, with varying 

 dilutions of B. atrosepticus. The results convinced the writer 

 that, unless the seed piece is infected at planting time there is 

 little chance that uninjured plants will contract the disease even 

 though the causal organism is washed about the stem and root 

 system. 



