May 27. 1918 Pox oj Swect Potato 441 



Infection may also be obtained by placing a healthy root whose surface 

 had been sterilized and then placed in contact with the slime developed 

 from a pox spot in the moist chamber, as described above. The ease 

 with which the slime organism may be produced from infected 

 potatoes in the moist chamber should make it excellent mate- 

 rial for class work. There seems no doubt that the slime mold Cystospora 

 batata is the cause of pox. To prove this definitely, the writer has carried 

 out the following experiment : The slime mold was produced aseptically 

 from young pox spots, as previously described. Bits of the slime were 

 transferred into the tubes of melted and properly cooled agar-agar, and 

 poured into sterile plate dishes. The plates proved sterile, showing that 

 there were no associated organisms in the slime which might be suspected 

 of being the cause of pox. Cysiospara batata refused to grow on petri 

 plates in hard agar. This explains why the writer had failed in previous 

 attempts to locate the causal organism of pox. In the larger number of 

 plate cultures made, the organism never appeared from the infected 

 tissue. This delayed the progress of the work. C. batata, how- 

 ever, may be grown artificially by the following method, suggested by 

 Dr. T. F. Manns, ^ of the Delaware Experiment Station: 



The Plasmodium may be grown upon a rich medium of ground up sweet potato, In 

 which about five grams of agar is added per one thousand c. c. Use about 500 grams 

 of such ground up sweet potato in a thousand c. c. of v/ater, leaving as much food as 

 possible in the medium, when made up to 1,000 c. c. 



By taking out asceptically bits of tissue from young pox spots, and 

 placing them in a flask in the above medium (sterilized in the autoclave 

 at 1 5 pounds' pressure for 1 2 minutes) , the organism will make fair growth. 

 To establish further and more definitely the pathogenicity of Cysiospara 

 batata, six 7-inch pots were filled with typical sweet-potato soil, and 

 sterilized in the autoclave for 12 hours at 20 pounds' pressure. After 

 cooling dovN^n, four of these pots were inoculated on June i, 1917, with 

 the slime grown artificially on the sweet-potato medium in the flask, 

 and the remaining two were left as controls. The inoculum was poured 

 into the four pots and thoroughly mixed in the soil with a sterile spatula. 

 All of the six pots were planted with healthy sweet-potato sprouts, two 

 of the latter to each pot. The pots were kept in the laboratory where 

 there was no possible chance for contamination. After three months all 

 the pots were emptied and the roots in each carefully examined. The 

 roots in the four pots which had been artificially inoculated with the slime 

 gro\\Ti in flasks showed definite symptoms of pox, whereas the roots in 

 the two control pots were all healthy. This seemed to prove beyond 

 any reasonable doubt that C. batata is the cause of pox of the sweet 

 potato. Negative results were always obtained when bits of tissue 

 taken from a pox spot where inoculated into a healthy sweet potato, 



' Correspondence dated October 26, 1916. 



