244. 
Journal of Agricultural Research 
Vol. VII, No. s 
only 32 of the sound tubers, or about 11 per cent, showed signs of decay, 
while 188 of the infected tubers, or about 58 per cent, showed the rot. 
About 3 per cent of the former and about 58 per cent of the latter were 
worthless for table use. 
Although interesting, the data gathered from the crop of 1913 was 
not definite, owing to the fact that the conditions under which the 
potatoes were grown and stored were unknown. More exact data were 
obtained from the crop of 1914, which was grown on 15 acres of land 
located in a field in which 75 per cent of the hills had produced tubers 
infected with Spongospora subterranea. From this 15-acre plot 16 
barrels were collected on October 7, as they were being harvested, and 
placed in good storage at Caribou the following day. The amount of 
infection shown by these potatoes varied from only a few sori to enough 
literally to cover the tuber. On June 15, 1915, parts of 7 of the 16 
barrels were sorted over, and the potatoes showing powdery-scab but 
no rot were separated from those showing rot. Table XIV gives the 
results of this study. 
TablK XIV. — Occurrence of powdery-scab dryrot on the crop of IQ14 
Barrel No. 
1 
2 
3 
4 
5 
6 
Number 
of bushels 
examined. 
Number 
of tubers 
free from 
powdery- 
scab. 
Number 
of tubers 
showing 
powdery- 
scab. 
Percentage 
of tubers 
showing 
powdery- 
scab. 
1 
70 
124 
63-9 
sK 
170 
173 
50.1 
2 X 
190 
368 
65-9 
1 
92 
124 
59 -6 
I % 
238 
149 
40. I 
1% 
38 
I 75 
55-9 
As will be seen from Table XIV, from 38 to 65.9 per cent of the tubers 
infected with powdery-scab rotted to such an extent as to be unfit for 
table use, and this notwithstanding the fact that these potatoes were 
harvested when fully mature and placed in good storage at once, as 
already stated. 
SYMPTOMS OF POWDERY-SCAB DRYROT 
The relation of Spongospora subterranea to powdery-scab dryrot is in 
a way comparable to the rot which follows Phytophthora infestans. S. 
subterranea causes little rot, but it leaves an open wound, through which 
wound parasites may enter. It is well known that P. infestans causes 
little rot unless it is followed by bacteria and wound parasites. 
The symptoms of the dryrot following infection by Spongospora sub¬ 
terranea may vary greatly, according to the time of year, storage condi¬ 
tions, state of the tuber when harvested, and the stage of development 
of the sorus when the infected tuber is removed from the soil. In 
