56 BULLETIN 64, XJ. S. DEPABTMENT OF AGEICULTURE. 
It is not thought worth while to reproduce here the tables and sum- 
maries of these variety tests. The varieties grown in Europe are 
almost entirely different from those grown in the United States, and 
repeated experiences have shown that few of them will thrive here if 
introduced. In general, the indications are that varietal differences 
in susceptibility to leaf-roll do exist, but that the tests need to be 
carried on longer before any conclusions are drawn respecting given 
varieties. It seems certain that leaf-roll is not a result of "running 
out" of varieties through old age, for many quite recently originated 
strains are affected. More striking still is its occurrence in seedlings, 
which has been observed by several workers. 
There have been unusual opportunities to study the occurrence of 
leaf-roll in the Stuart collection of 10,000 seedlings, where perhaps 
the most striking feature was that the leaf-roll was confined to certain 
numbers. The five hills of a kind would be uniformly affected, while 
those on either side were perfectly healthy. Clearly, the disease is 
not due entirely to soil or climatic influences, and certainly there was 
no indication of fungous infection. The marked contrast between 
diseased and healthy rows is well shown in Plate VI, in which the 
left-hand row is a hybrid (No. 2171) between Sophie and Keeper, the 
healthy row on the right being from the same cross (No. 2165). 
An interesting suggestion is put forward by Hedlund (1910), that 
leaf-roll is a pathological, adaptative mutation, and, further, that since 
acquired characters are not inherited the leaf-roll character must be 
latent in normal potatoes. 
CONTROL OP LEAF-ROLL. 
No measure offers more hope of success in controlling leaf-roll than 
the use of better seed stocks. Three means may be used to bring 
this about: First and simplest, the importation of seed potatoes from 
districts where the disease is unknown. This affords relief but may 
not greatly raise the standard of quality. Second, hill selection, to 
pick out from weak varieties strains that will withstand the disease. 
This has been done already by Von Lochow (1910), who took several 
types from the variety Professor Wohltmann and bred them in pure 
lines. The result was that certain of these pure strains showed sus- 
ceptibility to leaf-roll, while others remained entirely or nearly free 
from it. Third, new varieties may be bred from seed. This, while 
requiring the most time, may be the best means for meeting the 
requirements where whole districts are attacked, as in the Colorado 
outbreak. That such good varieties can be produced one can hardly 
doubt after seeing the departmental collection of over 10,000 seed- 
lings with its infinite variety of disease-resistant qualities. 
It is the prevailing opinion of European investigators that leaf-roll 
is inherited — i. e., that the tubers from diseased hills will produce 
diseased progeny. Cases are cited where the first crop after the 
