266 Journal of Agricultural Research voi. iv, no. 3 



Osbom (14) was the first to make an extended cytological study of 5. 

 suhterranea and described for the first time the formation of plasmodia 

 from separate amoebae within the host cell. He also discusses vegetative 

 nuclear divisions, nuclear fusions, and supposed reduction divisions, but 

 his description, as well as his figures, gives the impression that he has been 

 rather free in interpreting what he observed. Osbom (14) saw the 

 division of infected host cells and gives this as the only method by which 

 the disease spreads in the tissues. He describes a single uninucleate 

 amoeba in a young potato cell as the earliest stage with which he is 

 acquainted. This part of his description agrees with observations pre- 

 viously made by Home (5). 



In the early spring of 191 3 Giissow (4) reports having received speci- 

 mens of 5. suhterranea from Quebec and other provinces of Canada. 

 This is the first account of the disease in North America. During 191 3 

 powdery scab was found in one of our chief potato sections by Melhus 

 (12), who reported it on the 191 2 crop of northem Maine. This section 

 of Maine joins the Province of New Bmnswick, Canada. The presence of 

 Spongospora in the United States raises the question as to the effect 

 this introduced parasite will have on our potato industry in the various 

 parts of the country. This paper deals primarily with certain funda- 

 mental facts relating to the life history of the organism and its parasitic 

 relations to the potato, which, as indicated above, are very imperfectly 

 understood. In the author's studies of the germination of the spore 

 balls of S. suhterranea it has been found that a large number of small 

 Plasmodia are produced by the fusion of the amoebae on certain cultural 

 media. These results, coupled with previous observations in microtome 

 sections of very young sori, show unmistakably that infection takes place 

 through invasion by a plasmodium. 



INFECTION OF YOUNG TUBERS 



During the summer of 1913, while stationed at a field laboratory 

 at Caribou, Me., the writer was able to obtain abundant material of 

 5. suhterranea. Various stages of the disease on young tubers, were 

 fixed in Flemming's solution. This material was embedded in paraffin in 

 the usual way and has been the source of most of the sections from which 

 this study was made. Material for the study of spore germination was 

 obtained from the crops of both 1913 and 1914. 



On August 20 some young potato tubers were brought into the labora- 

 tory showing brownish-colored blisters easily recognized as very early 

 stages of 5. suhterranea. Others showed none of these blisters, but instead 

 very small, inconspicuous, light-brown-colored circular spots. These spots 

 were never more, and usually less, than % mm. in diameter. On careful 

 observ^ation it could be seen that each of the tiny brownish spots was 

 surrounded by a circular translucent area that varied from i to 2 mm. 

 in diameter. In most cases its limits could easily be distinguished. The 

 light, brownish spot at or very near the center of the translucent area was 



