July i, 1918 
True Nature of Spinach-Bligkt 
45 
dition of the diseased plant at the time it was used as food by the original 
ancestors; the temperature and humidity which prevailed both during 
the period when the aphids were feeding on the diseased plant and when 
their descendants fed on the healthy plants; and the parts of the plants 
on which the aphids fed. It is of importance to note in this connection 
that many aphids, although they may have fed on a diseased plant, are 
not necessarily virus carriers. From the data collected in the green¬ 
house and insectary experiments it appears that not over 50 or 60 per 
cent of the virus-bearing aphids produced blight in the healthy plants 
to which they were transferred. likewise, many plants show a tempo¬ 
rary immunity to the disease. In several of the earlier series plants 
were inoculated with a known virus and remained healthy for 72 days, 
at which time they were reinoculated. Infections occurred, and positive 
symptoms of blight appeared 17 days later. 
Since it is possible under certain conditions for aphids to transmit the 
causal factor of spinach-blight to their offspring, thereby enabling the 
latter to produce infections in healthy plants on which they feed, it is of 
interest to note that, while it indicated a closer association between a 
plant pathogene and its transmitter than has hitherto been known to exist, 
yet there occur among animal and human diseases several instances 
where the relationships between the definitive parasites and their trans¬ 
mitters are similar to those found with spinach-blight. 
For many years it has been known that certain animal diseases are 
communicable by the transference of virus from diseased to healthy indi¬ 
viduals. The viruses are usually highly infectious and, although filter¬ 
able, are thought to contain elements of a parasitic nature which thrive 
and reproduce in one or more hosts. Many zoologists have considered 
that these ultramicroscopic, infectious entities are probably protozoans. 
Several of the more important human diseases belong to the group caused 
by filterable virus. Little is known concerning their nature, except in 
some cases the method of their transmission, their alternate hosts, and 
various points in their life history. Knowledge of the definitive organism 
is lacking in each case. 
Recently it has been found that there is a group of plant diseases that 
are caused by filterable virus. Studies of certain of these diseases indi¬ 
cate them to be of parasitic origin, and from the nature of spinach- 
blight—namely, that it is caused by an infectious virus, and that no 
microscopic organism has been found associated with the disease—it con¬ 
stitutes another addition to this peculiar group of plant diseases. 1 
1 Through the courtesy of Dr. L. A. Hawkins, of the Bureau of Plant Industry, the juice of diseased 
spinach plants which had been filtered through a Berkefeld filter and a porous clay cup was obtained for 
inoculation purposes. A limited number of inoculations were made, but unfavorable cultural conditions 
caused the inoculated plants to yield rather uncertain results; therefore it seems advisable to repeat these 
expe rimen ts under more favorable conditions before the results are published. 
