oct. 30,1916 Spongospora subterranea and Phoma tuberosa 
225 
tuber tissue filled with spore balls of the fungus. The diseased tissue 
gradually dies, disintegrates, and liberates the spore balls, the sorus at 
this time being mature. The earliest infections on the tuber usually occur 
about the stem end, but as the host matures pustules may develop about 
the eye end. 
The symptoms do not always agree with the foregoing, but may vary 
in accordance with external conditions. For instance, it would seem that 
if a tuber were detached from the mother plant at the time infection is 
becoming visible and immediately placed in a moist chamber the devel¬ 
opment of the pustule should continue; but this is not the case, for while 
the infected area may increase a little, its developmentis seriously checked. 
Again, if the stems are cut off at the surface of the soil in the early stages 
of infection, the formation of a pustule is checked, but the discolored 
area may increase and the epidermis may be raised slightly, which symp¬ 
toms suggest that the plasmodium continues to vegetate rather than to 
break up and form spore balls. Two rows from which the tops were 
removed on August 11, six days after the first infection was found on one 
of the plots, showed only 3 per cent infection, while the checks showed 24 
per cent, but the former showed a reduction of about 75 per cent in yield. 
In like manner, if plants are attacked by lateblight shortly after infection 
with Spongospora subterranea begins to appear, further infection is 
stopped, and that which has already begun makes but little progress. 
This occurred in many fields in Maine during the season of 1915. The 
severe outbreak of lateblight checked the development of the potato 
crop and of powdery-scab. On the other hand, if the potato plant 
continues to grow after infection has once taken place and external 
conditions are favorable for the development of 5 . subterranea , the sori 
increase in depth and diameter (PI. 10, fig. C), which suggests the canker 
stage of 5 . subterranea known in Ireland. The tuber shown in Plate 10, 
figure C, is the progeny of a plant that grew in the greenhouse and did not 
mature and die down until 152 days after planting, although its tubers 
showed infection after 76 days. The unusually large size of the sori in 
this case is attributed to the long growing period of the plant, which was 
about 40 days longer than usual. Such sori are common where potato 
plants are grown in wet, infected soil in the greenhouse and would doubt¬ 
less result in the field if the crop had a longer growing season. In most 
potato-growing sections along the Atlantic seaboard only about 100 days 
are allowed, this being especially true in northern Maine. 
These greenhouse experiments suggest that the absence of the canker 
stage in the United States may be due to the short growing period for the 
potato plant in districts in which Spongospora subterranea thrives. 
A careful examination of material showing the canker stage as it exists 
in Ireland 1 (PL 10, fig. E) proved that it differs from the prevailing type 
of scab caused by Spongospora subterranea prevalent in this country. 
1 This material was obtained through the kindness of Dr. George H. Pethybridge. 
