Br.ACKi,E;G. 313 



tissues show a soft, wet decay. Preparations made from the 

 tissues that are just being invaded, and examined with sufficient 

 magnification show them to be filled with motile bacteria. 



Ususally the seed tubers attached to affected stems are entirely 

 decayed by a soft rot, or have disappeared entirely, while those 

 attached to surrounding healthy plants are generally quite firm. 

 If young tubers have been formed before the complete invasion 

 of the stem they are occasionally affected in the same manner, 

 although, as a rule, there is a tendency for the disease not to 

 follows out upon the branches which bear the tubers but upward 

 on the main stem toward the surface. Apparently the disease 

 works more rapidly or attacks the plants as a rule at an earlier 

 stage in their growth than the Rhizoctonia or "potato rosette" 

 disease described by Rolfs in Colorado and Selby in Ohio,* 

 for there is less tendency to produce little potatoes as there 

 described. Occasionally when the disease makes slow progress 

 on account of dry weather this tendency to throw out new 

 shoots above the affected region bearing many small potatoes 

 has been observed, even to the extent of producing small green 

 tubers upon the stem above ground. 



Out of a large number of affected fields examined only one 

 indicated possible spreading in the field. This was in 1907. 

 There was very little blackleg in the entire field of 20 acres 

 except in one spot a few rods square where all the plants were 

 diseased. It was first noticed near the center and gradually 

 worked outward. The season was excessively wet, and the 

 .affected area coincided with a low pocket or depression in the 

 field where water would stand for a few hours after each heavy 

 rainfall, thus indicating how, in this exceptional case, the disease 

 spread from hill to hill. In all other cases observed affected 

 plants were scattered promiscuously over the field, always 



decay of the aerial portions of stem without discoloration. Several cul- 

 tures were obtained from colonies on plates poured from this stem. 

 In every case tried these have, when inoculated into plants in the green- 

 house, produced not the colorless decay of the stem but the characteris- 

 tic blackleg decay. It may be said, however, that the bacteriological 

 studies upon this strain, so far as made, indicate greater variation from 

 the published description of B. solanisapnis than others being studied. 



* Rolfs, F. M., Col. Exp. Sta., Bui. 70 (1902) and Bnl. 91 (1904). 



Selby, A. D., Ohio Exp. Sta. Bui. 139 (1903). 



