Jan. 27,1923 
Relation of Potato Skinspot to Powdery Scab 
287 
by Pratt (9, PI. 34, fig. 2-3), and also the occasional presence of Fusa- 
rium spp. in the pustules. 
None of the workers mentioned in the preceding paragraphs gave 
conclusive proof for the support of their respective opinions as to the 
cause of skinspot. Their conclusions were based mainly on the presence of 
fungus hyphae within the dead cells and on subsequent cultures of the 
diseased tissues. The predominant organism in these cultures was 
assumed to be the cause of the trouble. According to the work of Owen 
(7) it was apparently Oospora pustulans in England, while according to 
the account of Wollenweber (jo), it was evidently Phoma eupyrena in 
Germany. Giissow ( 2 ) and O’Brien ( 6 ) report no experimental work 
whatever. Carruthers (i) and Pethybridge ( 8 ) confined themselves to 
microscopic observations only. Milbum and Bessey (5) made inocula¬ 
tions of healthy tubers with Spicaria nivea but failed to reproduce the 
disease. Wollenweber (jo) is content that he produced “similar spots” 
with P. eupyrena, but we know that an insertion of bits of mycelium of 
almost any facultative parasite in the needle pricks of tubers may result 
in the formation of brown pimples, externally resembling many a similar 
pathological condition. He gives no photographs and no description of 
the artificially produced Phoma spots by which others could judge as to 
the extent of their similarity with the skinspot pustules. 
Owen (7) conducted a series of experiments both in the laboratory and 
in the field, which are described in sufficient detail. The results, however, 
were either entirely negative or not decisive. Only in one experiment a 
“small number” of tubers inoculated in the field developed the typical 
skinspot after they were stored in a cellar from lifting time until April. 
It should be noted that this experiment was carried on in unsterilized 
soil in the field, and that— 
much blight was present and a large number of the tubers had to be discarded. 
It is not at all impossible that the latter fact may account for the absence 
of spots in one of the inoculated varieties and, perhaps, in the controls 
as well. On the other hand, the presence of “a few positive results” 
which became evident only in April after several months of storage in a 
cellar can hardly be regarded as proof that this small subsequent develop¬ 
ment was actually due to the artificial inoculation in July of the preceding 
year. The possibility of contamination either in the field during the 
growing period or later in storage is not at all excluded. It is not sur¬ 
prising, therefore, that the author arrives at the conslusion that— 
it will be necessary to wait another season before the results can be confirmed or 
amplified. 
As yet, no further reports have been issued and the work, apparently, 
has been discontinued, at least temporarily. 
TRUE NATURE OF SKINSPOT 
While all the alleged causes of the skinspot disease lack convincing 
proof, a comparative examination of a large number of the affected tubers 
cannot fail to disclose a great similarity of these spots to the closed- 
pustule stage of powdery scab. The filamentous fungi invading the 
tissues of the pustules unquestionably complicate the conditions. Yet, 
whatever is the effect of this subsequent invasion, there is sufficient 
evidence that the original cause of this injury is the same as in powdery 
scab, ascribed to Spongospora subterranea (Wallr.) Johns. This evidence 
is presented hereunder. 
