292 
Journal of Agricultural Research 
Vol. XXIII, No. 4 
signify their inability to secure this organism in culture, were it always 
present in the diseased tissues. It is merely a reflection of the actual 
state of things. Their conclusions were determined mainly by the prev¬ 
alence of one or the other semisaprophytic fungus in their cultures or by 
the absence of any apparent growth, as the case might have been. So 
far as the primary cause of the trouble is concerned, they were dealing 
with an organism which is far from being readily culturable by our usual 
laboratory methods. 
VARIOUS STAGES ON THE SAME TUBER 
While it is possible to single out many tubers which are covered only 
with the closed pimples to which the name skinspot is applied, yet in the 
examination of large quantities of infected material one is likely to find 
a certain number of specimens, varying perhaps with the seasonal condi¬ 
tions, which show all gradations in the development of the sori. It is 
rather difficult and sometimes impossible to determine whether the 
closed pustules in such cases should be classed as skinspot or powdery 
scab (PI. 1, E to H). Like difficulty may be encountered in an examina¬ 
tion of some unquestioned powdery scab material, grown, for instance, 
in Maine or Canada (Pl. 3, A, B, C). 
An interesting question arises as to why in certain years there is an 
abundance of immature sori of the Spongospora scab. Theoretically 
speaking, this condition may be due either to an early check in the develop¬ 
ment of the disease or to a late infection. Both may be brought about 
by drouth and aided by the varietal response of the tubers. It is probable 
that late varieties with a later formation of tubers are more likely to show 
undeveloped powdery scab pustules, especially under adverse conditions. 
During the latter part of September, 1920, the writer visited Sea Island 
and Lulu Island in British Columbia. A number of fields were examined 
on which it was said skinspot infested crops were produced in the 
preceding years, but no signs of the disease were noticed. Neither was 
the well-developed stage of powdery scab common. The local workers 
explained that the last two seasons had been very dry and in consequence 
powdery scab was practically eliminated. On the other hand, 1919 was 
the year when skinspot made its pronounced appearance and was noticed 
in abundance by United States inspectors and pathologists. 
An early stage of Spongospora infection is usually not conspicuous at 
harvesting. The infected spots are not dark in color until they are 
exposed to the drying which takes place after lifting. During the long 
period of storage the tubers gradually lose moisture and shrink. The 
hard, dry, dark pustules then become elevated, especially when they 
become invaded by saprophytic or semisaprophytic organisms. 
DISCOLORATION AND ARRANGEMENT OF THE SKINSPOT PUSTULES 
Upon lifting the epidermis the majority of the spots show a peculiar 
chocolate-brown or olive-brown discoloration of the affected areas. 
This discoloration has frequently been described in the literature as very 
characteristic of powdery scab. It is not known to be so constantly as^ 
sociated with any other disease of the potato tuber. The arrangement 
of the pustules over the skin is likewise very suggestive of the Spon¬ 
gospora scab. Although often scattered without any definite order (PI. 
2, D), as is also the case with indisputable specimens of powdery scab 
