222 MAINE AGRICULTURAI, JjXPijRIMENT STATION. I9II. 



show that the disease had been carried over in the soil and the 

 growing crop infected by that means. On the other hand the 

 appearance of the disease on fields not planted with potatoes 

 for years or never before planted with any agricultural crop, 

 and on farms for the first time following the use of seed from 

 a difl^erent source all indicate the seed tubers as the source of 

 infection. Moreover, as has already been mentioned, the disease 

 does not occur in patches but in scattered hills all over a given 

 field, and always begins at the base of the stalk where it joins 

 Avith the seed piece, which latter is invariably destroyed by a 

 soft rot. 



Two of the experimental fields used this season gave very 

 positive evidence in support of this view. Mr. Donaldson's 

 Ir'ish Cobbler field and Mr. Hussey's Green Mountain field were 

 both planted the second time in succession. In both cases the 

 disease appeared on the field in considerable amounts in 1910 

 and a part of the crop there produced was used for seed pur- 

 poses in 191 1. Where the seed was treated it was practically 

 eliminated on Mr. Donaldson's entire field of 50 acres. On Mr. 

 Hussey's field no diseased plants could be found where the 

 treated seed was used and 163 per acre appeared where the 

 seed was planted without disinfection. On such fields there are 

 always quite a percentage of "volunteer" plants which spring 

 from tubers which were either not brought to the surface or 

 were covered up by the digger and remained in the soil all win- 

 ter. These are frequently easily recognized on account of their 

 springing up irregularly on the sides of the rows or between 

 the hills. On the two fields mentioned not a plant of this kind 

 was observed to be attacked with the disease. 



Infection of Healthy Seed-pieces at or Foleowing 



Cutting. 



The bacteria causing this disease multiply fairly rapidly in 

 the presence of a sufficient food and moisture supply at tem- 

 peratures of from 65° to 75° F. and with great rapidity at from 

 76° to 85° F. At the temperatures usually maintained in the 

 potato houses for winter storage — often but a few degrees 

 above freezing — they multiply very slowly but are capable of 

 remaining alive under these conditions for considerable periods 

 of time, provided they are not allowed to dry out. Doubtless 



