oct. 3o ( 1916 Spongospora subterranea and Phoma tuberosa 
245 
general, however, the symptoms may be divided into three groups: (1) 
Desiccation or loss of moisture from the wound caused by S. subterranea , 
(2) those caused by the germination of spore balls in the bottom of the 
sorus and the formation of the plasmodium which destroys the adjoining 
cells, and (3) those caused by the entrance of wound parasites into the 
sori. Often, however, all three causes work together, and in many cases 
the fungi begin their work after some desiccation and plasmodium injury 
have set in. It should also be borne in mind that dryrot does not occur 
about every sorus of a tuber infected with 5. subterranea . A tuber may 
be well sprinkled with sori and yet show no dryrot; or the rot may 
appear about a few of the sori, this latter being true in the case of 
infected tubers harvested both when immature and when quite ripe and 
filled with countless spore balls. 
Powdery-scab dryrot due to the desiccation of the tissues adjoining 
the sorus results in discoloration, shriveling, and shrinkage. When 
infested tubers are harvested, the sori are virtually open wounds, and 
when placed in warm, dry storage the temperatures incident to which 
often prevail in early fall, desiccation of the living cells bordering on the 
wound takes place. When the storage temperature drops with the 
advance of the season, this type of dryrot is retarded. It can be induced 
readily, however, by taking tubers from storage and holding them for 
a fortnight at ordinary room temperature. The cause of the desicca¬ 
tion of the cells about a sorus is readily seen in sections of the sori made 
before and after shrinkage takes place. Plate 12, figure A, shows the con¬ 
dition in the bottom of a sorus before there is any appreciable amount 
of desiccation. It should be especially noted that no cork has formed 
in the bottom of the pit, the adjoining living cells being protected by 
the dead debris and spore balls of the sorus. By referring to Plate 14, 
figure A, it will be seen that abundant cork cells are formed in connec¬ 
tion with a sorus of common scab. This explains why dryrot is very 
seldom or never associated with this disease, 
A comparison of figures A in Plates 12 and 14 shows clearly why desic¬ 
cation might take place in one case and not in the other. 
The condition of a sorus after some dryrot due to desiccation has 
taken place is shown in Plate 12, figure B. When the material was 
dehydrated, the spore balls of the fungus were washed out. It is espe¬ 
cially interesting to note the number of dead cells that are still partly 
intact. There are also signs of the formation of some wound cork. 
Where this was found, the underlying cells were still alive. There was 
no evidence in this sorus of a plasmodium, such as occurs in connection 
with dryrot due to plasmodium injury. The type of dryrot due to desic¬ 
cation never causes serious damage, but it does further mar the appear¬ 
ance of tubers already injured by the sori of Spongospora subterranea. 
The second type of dryrot is that caused by the plasmodium of Spongo¬ 
spora subterranea . The spore balls in the bottom of the sorus germinate 
