112 
Journal of Agricultural Research 
Vol. XXV, No. 2 
same variety. It is possible that differences in performance between 
strains of the same variety result from varying percentages of those 
degeneration diseases and not merely from other variations, such as 
vigor, as is frequently designated. Disease-free strains of the same 
variety should be grown under absolutely uniform conditions for several 
seasons in order to permit accurate testing of the supposed inherent dif¬ 
ferences between strains of a variety. 
Regarding the general effect of the region the various experiments 
detailed in this paper tend to show that the potato degeneration problem 
is similar in the northeastern United States to the same problem in other 
parts of America (including Canada) and in parts of Europe (including 
the British Isles). It is a complex problem, involving several diseases 
that react differently to transmission agents, varieties, and other envi¬ 
ronmental factors. While this general similarity is true, it is also true 
that the problem because of its complexity may vary greatly from one 
locality to another. It follows then that control measures must be 
worked out for different sets of conditions, following research based 
initially on the general principles now fairly well understood. 
Two examples may be given of striking differences between regional 
problems. These may or may not be referable fundamentally to climate, 
bearing in mind that the preference given to varieties in a given region 
or the prevalence of insects transmitting a disease may be determined 
by climate. The presence of the spindling-tuber disease in at least 
five widely separated States of this country, with its apparent absence 
from Europe, is a difference that, regardless of its cause, complicates 
the situation because of the elusiveness of vine symptoms. The presence 
of net necrosis as a tuber symptom of leaf roll in at least two States of 
this country and in Japan also is a feature that has not been noted in 
Europe. A third difference, consisting of the successful development 
and use of a leaf-mutilation method of inoculation in at least three 
parts of this country may disappear when the method is tried in other 
countries (p. 54). In view of such differences and the differences reported 
for parts of the same country, the effectiveness of certain control meas¬ 
ures in one region can not be trusted as a reliable indication of their 
usefulness in another region. 
With the degeneration problem showing somewhat different aspects in 
one region from another, with seed being transported from northern to 
southern regions to reduce injury from degeneration, and with the 
need for the use of all scientific results, it is desirable that it at least be 
possible- to identify a given degeneration disease in different places. 
This is not yet possible because of the modifying effects of climate and 
variety upon the symptoms, and because of the variation of climate and 
varietal preference from one region to another. Growing parts of the 
same tuber, with many tubers, and in different regions, is helpful but 
not always practicable. Undivided tubers are not reliable for comparison 
unless they are produced under conditions where no infection of the 
parent vines is possible. Great usefulness is in store for methods that will 
readily identify the yet unknown causal agencies, whether organisms 
or compounds, of the several diseases, somewdiat as phloem-necrosis 
microanalysis has been used by Quanjer (39, p. i3 2 ~37) to detect 
an apparently consistent symptom of leaf roll. Furthermore, it is 
quite apparent that the identification of the causal agencies, whether 
by analysis or by culture, will ultimately result in an authentic classi¬ 
fication of the so-called virus diseases. 
