242 
Journal of Agricultural Research 
Vol. VII, No. 5 
While it is not an uncommon thing to find powdery-scab-infected 
tubers entirely decayed, the rot generally occurs in spots. The spots vary 
from i to io cm. in diameter. They may be only slight depressions in 
the superficial layers, or they may extend into the center of the tuber. 
In this respect powdery-scab differs markedly from the scab caused by 
Oospora scabies , with which it may be easily confused before the dryrot 
sets in. The rotting of the tubers following infection by S. subterraned 
not only distinguishes this disease from that caused from O. scabies , but 
emphasizes its more destructive nature. 
Some idea of the prevalence and destructiveness of powdery-scab dry- 
rot can be formed from the percentage of this rot on tubers grown during 
the season of 1913 infected by Spongospora subterranea. Infected 
potatoes were collected from individual growers at different points in 
Aroostook County between May 2 and June 15, 1914. These were washed 
and separated into two lots, those showing dryrot and those free from 
the rot but infected with powdery-scab. Later the lots were weighed, 
and the percentage proportion by weight of those showing the rot is given 
in Table XII, together with other data regarding the tubers used. 
Table XII .—Percentage of powdery-scab dryrot on potatoes of the IQI3 crop infected 
with Spongospora subterranea 
Date of 
collection. 
Place of collection. 
Variety. 
Quantity 
collected. 
Percent¬ 
age of 
powdery- 
scab dry- 
rot. 
May 2 
May 5 
May 5 
May 21 
June 10 
June 15 
Patten , Me. 
Green Mountain. 
Bushels. 
6 
73 
30 
33 
67 
45 
35 
Presque Isle, Me . 
Irish Cobbler. 
7 
Caribou, Me . 
Green Mountain. 
O 
8 
Patten, Me. 
.do. 
5 
12 
Caribou, Me. 
.do. 
Ashland, Me. 
Irish Cobbler, Green 
5 
Mountain. 
As will be seen from Table XII, nearly 50 per cent of the tubers showed 
the dry-rot stage, the percentage of rot on the two varieties ranging from 
30 to 73. 
Although limited, these observations indicate that the Green Mountain 
is more severely attacked than the Irish Cobbler. This may be due to 
varietal differences, but it seems more logical to believe that it is due to 
the fact that the Green Mountain is a later-maturing variety. 
The rot was most prevalent on the tubers infected with the scabby 
stage of Spongospora subterranea , and this explains the low percentage 
found on the two lots collected at Presque Isle and Caribou on May 5. 
A very high percentage of the tubers badly infected with the scabby 
stage showed the dryrot in some form. Not only does the scabby stage 
of powdery-scab mar and deface the tuber and render it objectionable in 
