Jan. 27,1923 
Relation of Potato Skinspot to Powdery Scab 
289 
The formation of new cork in the skinspot pustules on the border line 
of the diseased and the healthy tissues is also very significant and con¬ 
tradictory to the supposed progress of the disease. This process of cork 
formation is not invariably associated with the spots, though it may be 
seen in a considerable number of the specimens. Owen (7) gives a draw¬ 
ing of a section through a pustule of skinspot which shows distinct layers 
of new cork below the affected areas. Doubtless this phenomenon signifies 
that the parasite, after killing several layers of cells unhindered in its 
attack, finally ceased its activities, after which the host tissues erected a 
barrier between the dead and healthy regions. It is the course diamet¬ 
rically opposite to that which we would expect in Oospora pustulans t which 
is said to be invisible at lifting time but develops a considerable injury 
toward spring, or about the month of April. It should, therefore, at 
this time of the year be in its most active stage and not in a dormant state. 
On the other hand, the phenomenon of cork formation is quite in harmony 
with the progress of powdery scab. The active parasitic stage of the 
latter develops in the field during the growing period of the tuber. As ob¬ 
served by Kunkel (3), a certain degree of saprophytic activity of the fun¬ 
gus plasmodium develops in storage. This secondary infection results in 
dry-rot areas around the sori. However, it is, apparently, very slight 
with some spots and may be entirely checked with others. The cicatri¬ 
sation in such cases is obvious. 
ABSENCE OF THE SKINSPOT FUNGUS 
During the years of 1920 and 1921 the writer had an opportunity to 
examine and study an abundance of material showing skinspot infection. 
This material was secured not only from England but also from other 
countries. It is believed, therefore, that a comparison of cultures isolated 
from this material will be of special value and interest. The various sam¬ 
ples from which isolations were made are as follows: 
1. Several tubers from British Columbia, crop of 1919, received in 
spring of 1920. Pustules only slightly raised above the surface of the 
tuber. 
2. Five tubers from Dr. A. D. Cotton, England, crop of 1919, received 
in spring of 1920; sent upon request as representative samples of the 
skinspot disease. 
3. Several lots of wart-resistant varieties shipped from England for 
work in Pennsylvania, crop of 1919, received in the winter of 1919-20. 
(PI. 1, A to H, and PI. 2, C and D, show some of these tubers.) 
4. Tubers from various consignments of Danish potatoes received at 
the port of New York, crop of 1919, collected in early spring of 1920, 
(PI. 2, B, shows one of these tubers.) 
5. A sample of potatoes from New Castle, England, secured by Federal 
Horticultural Board inspectors in the spring of 1920. 
6. A sample of potatoes from Edinburgh, Scotland, secured by Federal 
Horticultural Board inspectors in the spring of 1920. 
7. A sample of potatoes from Ireland, secured by Federal Horticultural 1 
Board inspectors in the spring of 1920. 
8. Two samples of the diseased tubers sent by Prof. J. W. Eastham, 
British Columbia, crop of 1920, received in the winter of 1920-21. 
9. A number of tubers from the progeny of the infected English va¬ 
rieties (sample No. 3) planted in Pennsylvania in 1920, isolations made in 
the spring of 1921. 
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