RELATION OF POTATO SKINSPOT TO POWDERY SCAB 1 
By Michael Shapovalov 
. Pathologist, Cotton , Truck , and Forage Crop Disease Investigations , Bureau of Plant 
Industry, United States Department of Agriculture 
INTRODUCTION 
The skinspot of the Irish potato tuber was first discovered, described, 
and named in England. It is said to be of considerable importance on seed 
potatoes, as the eyes may be injured and even killed by the causal organ¬ 
ism, and this obviously would result in uneven stands and reduced yields. 
The same disease has been reported from Canada, and an apparently iden¬ 
tical trouble has been described in Germany. It has also been noted 
on some shipments of Danish potatoes consigned to New York. Its 
presence in the United States was unknown until some wart-resistant 
varieties were imported from England and planted in the wart region 
of Pennsylvania. The progeny of these varieties showed a small per¬ 
centage of skinspot. Further influx of infected material to this country 
is very likely to occur as long as the importation of foreign potatoes 
continues. It is therefore important to ascertain the nature of this 
disease and the possibilities of its spread in the United States. 
SYMPTOMS AND ALLEGED CAUSES OF SKINSPOT 
Owen (7) 2 stated very correctly that the external characters of skin¬ 
spot are so distinctive that there is not much likelihood of confusing 
it with other spots. The appearance of the disease may be briefly de¬ 
scribed as follows: Round, raised, closed pustules frequently with a 
depressed border, single or in aggregates of varied configuration (PI. 
1, 2); dark brown or bluish brown outside and olive brown to brown 
inside. If the skin of the tuber is thick and rough, whether naturally 
or through the action of fungi or other causes, the spots are usually 
larger in diameter. Occasionally the pustules are lenticular or irregular 
in shape. Often the potato skin is attacked by Spondylocladium atro- 
virens, which gives it a glistening silvery appearance; but this is not 
characteristic of skinspot. In some cases the spots are not convex 
but are flat and only slightly raised and of considerably larger dimen¬ 
sions (PI, 1, B). The disease is so typical and peculiar that no difficulty 
has been experienced by workers in recognizing it either on specimens 
or on photographs. The nature of the trouble, on the other hand, is 
less obvious, and the opinions of various investigators on this point are 
greatly at variance. 
Carruthers (r) was the first to record what has been assumed to be, 
and presumably is, this disease. He found an abundance of fine myce¬ 
lium in dead cells of the pustules but did not identify the fungus. How¬ 
ever, his drawing of a diseased tuber covered with open pustules is 
very suggestive of powdery scab. No inoculation experiments are re¬ 
ported. 
1 Accepted for publication May 10,1922. 
s Reference is made by number (italic) to “ Literature cited," p. 294. 
Journal of Agricultural Research, 
Washington, D. C. 
acx 
(28s) 
Vol. XXIII. No. 4. 
Jan. 27, 1923 
Key No. G-273 
