Oct. 3 o, 1916 Spongospora subterranea and Phoma tuberosa 
219 
susceptibility of roots, stolons, and stems of potato plants 
TO SPONGOSPORA SUBTERRANEA 
Although Spongospora subterranea is known only as a disease of the 
tuber of the Irish potato, the fact that its causal organism is a slime 
mold and that many other species of the family flourish on the root 
systems of their hosts led the writers to suspect that it infects the 
other underground parts of the potato. Very meager information on 
this phase of the subject was obtained from a survey of the literature on 
the disease. Johnson (5) incidentally mentions seeing pustules on 
stolons and Pethybridge (12, p. 352) mentions evidence of the disease 
on roots and sprouts, but in no way do these references show the preva¬ 
lence and actual relationship. In the fall of 1914 a lot of 200 pounds of 
soil was collected in a field in which a crop of potatoes infected with 
S. subterranea had just been grown. Some of the soil was placed in 
12-inch pots in the greenhouse and planted with potatoes known to be 
free from powdery-scab. When the crop had been harvested, which 
was before all the vines were dead, and the root systems carefully washed 
so that they might be examined for any signs of infection, roots of all 
sizes were found to be very generally infected with white galls (PI. 7, 
fig. C, D) strongly resembling the well-known legume nodules. On 
sectioning these galls (PI. 8, fig. A) when nearly mature, they were found 
to contain a large number of immature spore balls of S. subterranea . 
Similar galls were found on the stolons and main stems of the plant 
(Pi. 7, fig. A, B), and in two of the pot cultures pustules were found on 
the stems about 1 % inches above the surface of the soil, this latter infec¬ 
tion having taken place probably while the soil covered this portion of 
the stem. 
In one case in which the galls on a single plant were counted, 149 were 
found on the roots, 19 on nine stolons, and 8 on three stems; besides, 
some were doubtless lost in the process of disentangling the roots. This 
plant produced nine tubers, four infected, and five free from Spongospora 
subterranea . 
The presence of Spongospora subterranea on the roots of potato plants 
growing in pot cultures in the greenhouse naturally raised the question as 
to whether infection is as prevalent under field conditions. Careful watch 
was kept for evidence of the disease on the roots of plants growing in 
the soil-treated plots on infected soil at Caribou during the summer 
of 1915. The first infection was not found until August 5, although 
examinations had been made weekly from the time the plants began to 
come up. At this time the galls, which were on the small rootlets both 
near the surface and deep down in the ground, were white and no larger 
than pinheads; but day by day they became more pronounced until 
finally they were comparable to those found in the greenhouse. No 
infections were found on the tubers when first noted on the roots, 
