July 14,1923 
Degeneration Diseases of Irish Potatoes 
85 
(39, p-130). Some of the differences in diagnosis and terminology may 
depend upon the use of different standard varieties used in Holland and 
America, respectively, for such comparison. 
INTERSPECIFIC TRANSMISSION OF MOSAIC INVOLVING POTATOES 
The definition of mosaic of potatoes, which apparently includes several 
distinct diseases, has been given (p. 46) and will serve for mosaic of other 
species of plants. It is not within the scope of this paper to give a review 
of the extensive literature on interspecific relationships of mosaic. It is 
sufficient to state here that many species have mosaic, that interspecific 
transmission from one taxonomic family to another is unusual in compari¬ 
son with that within such families, and that the mosaic diseases of various 
hosts, even in the same family, are by no means similar in behavior in 
regard to seed transmission, infectiousness, viability of the virus, and 
efficacy of different methods of inoculation. Experiments upon inter¬ 
specific transmission of mosaic to and from the potato have interest and 
importance. They indicate the relationship of potato mosaic to other 
types of mosaic that are better understood and so indicate by analogy 
the most promising improvements in control measures. They also fur¬ 
nish evidence upon the problem of alternate hosts which might serve to 
perpetuate and spread infection. 
The great variation in mosaic behavior indicates that in attempts at 
interspecific transmission, the methods used must be selected or modified 
to suit both species involved. With a given mosaic virus it is desirable 
to use both a method known to be successful in infecting the usual host 
and any methods that have been proved effective for a presumably differ¬ 
ent mosaic virus of the possible alternate host to be tested. In this way 
the experiment will take into account not only the possibility that the 
first virus is infectious to the alternate host, but also the possibility that 
it is infectious only in conditions different from those governing successful 
inoculation of the normal host. 
For example, if needle inoculation alone is used to transmit mosaic 
from tobacco to potato with observations ceasing in 15 days, it thereby 
apparently is assumed that the tobacco mosaic virus must act the same 
in all hosts. On the other hand, if it is deemed possible either that the 
tobacco virus may be responded to differently in the potato, or even that 
both mosaics are caused by the same virus with different behavior in 
different hosts, it is desirable to use methods that are reliable for both 
mosaics in the respective normal hosts. Another example is a needle 
inoculation of cucumbers with juice from mosaic potatoes, with obser¬ 
vations ending in 13 days. This method may test sufficiently the pos¬ 
sibility of the potato mosaic virus being the same as the cucumber mosaic 
virus, but it would ignore the possibility that the potato virus could be 
transmitted to cucumbers in the same conditions needed for successful 
inoculation within the normal host, the potato. Whenever interspecific 
transmission between two hosts is demonstrated it is still necessary to 
make further comparisons in regard to symptoms, susceptibility of hosts, 
ease of inoculation, and incubation period, in order to determine as nearly 
as possible whether one or more viruses are involved. 
POTATOES AND OTHER MEMBERS OF THE SOIyANACEAE 
In view of the requirements stated above, no completely adequate test 
has been reported showing that interspecific transmission between pota- 
