Jan 27, 1923 
Relation of Potato Skinspot to Powdery Scab 
291 
Nor is Phoma sp. alone in attacking the Spongospora sori in Maine. In 
addition to a certain proportion of sterile plantings, that is, those which 
yielded neither fungi nor bacteria, a considerable percentage of organisms 
other than O. pustulans was obtained from authentic skinspot material. 
Especially striking is the complete absence of the supposed skinspot 
fungus from the typical pustules of the disease developed on potatoes 
grown in Pennsylvania. The largest number of O. pustulans cultures 
was isolated from the tubers grown in the British Isles (69 out of the 
total number of 86 O. pustulans cultures, or 74.2 per cent, of all the plant* 
ings made from English material). A somewhat smaller proportion was 
obtained from Danish specimens and a still smaller one from the samples 
received from British Columbia. None was isolated from the skinspot 
pustules which developed on potatoes grown in Pennsylvania. 
It is not surprising that plantings of the diseased tissues did not yield 
Spongospora subterranea. So far only one worker (3) has reported 
success in culturing this organism; and, furthermore, these cultures 
were obtained by germinating the mature spore balls which are not yet 
present in the closed sori. 
That a considerable number of the true powdery scab sori are likely 
to become invaded by some secondary parasite is not a new discovery. 
Previous studies of this disease in Maine revealed the fact that even in 
good storage conditions 30 to 75 per cent of the tubers infected with 
powdery scab suffer from a secondary rot due to Phoma tuberosa. The 
colored plate accompanying the paper cited here (4) shows two tubers 
with a number of closed sori. The latter have a depressed border and 
a raised center corresponding to the characteristics noted in connection 
with skinspot in England; but the organism responsible, at least in part, 
for this condition in the latter country is apparently Oospora pustulans. 
In Germany, Wollenweber’s (jo) work indicates, that a similar effect is 
produced by a species of Phoma different from that occurring in Maine. 
The principal invader of the skinspot pustules in Pennsylvania appears 
to be Colletotrichum atramentarium . This predominance in this connec¬ 
tion of a certain species of fungus in a given country is an interesting 
phenomenon worthy of further study. It raises a question as to the 
r 61 e of climatic conditions in developing an intricate relationship within 
the fungus world, and in this case relationship between the powdery 
scab parasite and the secondary invader. It does not, however, solve 
the problem of the original cause of skinspot. It has long since become 
a commonplace in the science of plant pathology that the association 
of a certain fungus with the dead host tissue does not necessarily signify 
that it is the primary cause of death. It may as well denote any one 
of three other possibilities, namely, a secondary invasion of a parasitic 
nature, a saprophytic existence on the dead stratum, or a parasitic attack 
on the primary parasite. Fusarium spp. isolated by the writer from the 
skinspot pustules are, undoubtedly, secondary parasites, while such miscel¬ 
laneous fungi as Penicillium sp. or Actinomyces sp. probably live there 
saprophytically. The part which is actually played in the potato tissues 
by O . pustulans as well as that by Phoma eupyrena or C. atramentarium 
remains as yet to be shown. The writer's laboratory inoculations of 
healthy potato tubers with O . pustulans , like those reported by Owen (7), 
gave entirely negative results. 
It is not difficult to ‘culture Oospora pustulans whenever the spots are 
actually invaded by this fungus. Therefore, the disagreement of the 
various investigators as to the cause of skinspot does not by any means 
