Jan 27, ipaj 
Relation of Potato Skinspot to Powdery Scab 
293 
(PL 4, A), they are frequently arranged in a certain peculiarly regular 
manner, as for instance, in the form of bands or stripes (PL 1, A; Pl. 2, 
B, C), rings (PL 1, C, D) and aggregations, particularly at the stem 
end (PL 2, A; Pl. 3, A). All this is very typical of powdery scab (Pl. 4, 
B, C, D). 
GEOGRAPHICAL DISTRIBUTION OF SKINSPOT 
It is a striking “coincidence’’ that this disease occurs only in those 
countries, or in those sections of the countries, where climatic conditions 
favor the development of powdery scab. It has not yet been reported 
from those parts of the world where the latter is unknown. Thus, it is 
prevalent in the British Isles, exists in Denmark and Germany, and is 
established in parts of Canada. Occasional specimens showing closed 
pustules were observed in Maine, and they were always identified as the 
immature stage of powdery scab. It has developed recently to a slight 
degree in the elevated regions of Pennsylvania. No other similar trouble 
has-been discovered in any other section of the States in spite of a con¬ 
tinuous influx of skinspot infected potatoes from Europe. This is exactly 
the behavior of powdery scab. The writer planted a large quantity of 
Oospora pustulans material at Arlington Farm, Va., in 1920, but no spots 
were observed on new tubers either at harvest time or in the spring of 
1921. The same negative results were obtained with tubers planted in 
the greenhouse at Washington, D. C., and grown in various kinds of soil 
and under various conditions of moisture. Although the powdery scab 
sori may develop in Washington greenhouses, this is known to take place 
only when tubers with the mature spore balls are planted. It has not 
yet been ascertained whether the organism in the closed pustule stage 
can readily, if at all, become a source of infection of the new crop. 
SUMMARY AND CONCLUSIONS 
Evidence is submitted in the preceding paragraphs which show that: 
(1) Neither Oospora pustulans nor any other filamentous fungus or 
environmental conditions mentioned by previous authors in connection 
with skinspot are proved to be the primary cause of the disease. 
(2) Oospora pustulans is prevalent in England, and probably in some 
geographically proximal countries, but is either rare or entirely absent 
in the skinspot material grown in America. 
(3) The various peculiar characteristics of the skinspot disease, as well 
as its geographical distribution are fully in accord with our present 
knowledge of powdery scab. 
(4) Certain of these peculiar features can not be satisfactorily ex¬ 
plained apart from the identity of the two diseases. 
(5) The closed or immature sori of powdery scab present no apparent 
difference from the pustules of skinspot. 
(6) The fungi associated with the spots in question are mainly secondary 
invaders developing during the storage period. 
(7) These secondary invaders do not belong to one species, nor even 
to one genus, but vary with the country, their prevalence being deter¬ 
mined perhaps by climatic conditions and the flora of the soil. 
(8) Their presence in the sori of powdery scab is altogether unnecessary 
to give them the appearance of skinspot. 
This evidence leads us to the conclusion that the skinspot pustules are 
essentially and primarily the closed or immature sori of powdery scab. 
