236 
Journal of Agricultural Research 
Vol. VII, No. 5 
the development of the cork cambium and susceptibility is not improbable, 
and this is strongly indicated by the fact that probably 90 per cent of the 
pustules on the infected tubers from the experimental plots were about 
the stem ends rather than the eye ends. While this preponderance of 
infection about the stem end was true in 1915 on the experimental plots, 
the condition in this respect in other fields was not determined, and no 
such preponderance was noted during the previous season, when infection 
occurred much later, or between August 20 and about September 5. A 
critical examination of some of the varieties and of the seedlings attacked 
showed that not only the percentage of tubers infected but also the 
macroscopic character of the sori differ. 
Different varieties of potatoes respond differently to the attacks of the 
disease, as shown in Plate A, which represents various kinds of sori. It 
was possible to arrange a series from mature sori, varying from the size 
of pin pricks to that of 1 or 2 cm. as they occur on such standard varieties 
as Green Mountain and Irish Cobbler. The reaction of the host showing 
the smaller sori differed markedly from that of the host showing the 
larger sori. In some varieties the sorus attains a considerable size 
before it bursts, in others it breaks open very early, while in still others 
the infected area corresponding to the sorus shows only discoloration and 
gives the impression that the plasmodium is spread throughout this dis¬ 
colored area, but at no point does it cause sufficient proliferation of the 
host cells to form a definite open sorus. 
The data regarding the response of the different varieties lead the 
writers to believe that the response is not due entirely to resistance but 
rather to the fact that the tubers of certain varieties have escaped infec¬ 
tion, and furthermore that a variety which will show much less infection 
than the Green Mountain may be found. 
SOII, TREATMENT 
Five acres on one of the farms first cleared in the vicinity of Caribou 
and said to have been under cultivation for at least 30 years were used in 
1915 for the study of effects of soil type and soil treatment on the disease. 
Because of its close proximity to the village, this land has been cropped 
rather heavily to potatoes during the previous 15 years. 
Topographically the tract occupies the position of a high glacial river 
terrace sloping from west to east. Along the northeast side and in part 
forming the northern boundary is a well-marked drainage way or ravine 
in which considerable erosion has taken place, leaving the soil exceedingly 
stony and with less fine earth than elsewhere in the field. With the 
exception of this and a small area at the southwest comer, the slopes are 
not steep and there is little or no noticeable erosion. In the central part 
of the field the slope appears inconsiderable, but the instrumental deter¬ 
mination showed not less than 5 feet elevation in 100 horizontal feet, a 
