July 1, 1925 
Overwintering and Dissemination of Cucurbit Mosaic 
33 
mosaic disease on this host, and Clinton (4), in 1915, stated that he 
had been unable to transmit pokeweed mosaic to healthy tobacco 
plants. Allard (1), in 1914, also mentioned pokeweed mosaic and 
reported that it was not transmissible to tobacco. In a later paper 
(%), Allard showed that the disease was readily transmissible to 
healthy pokeweed plants, but stated that he was unable to transmit 
it to tobacco or pepper. One of the writers (Doolittle) ( 8 ) also 
reported that he had been unable to transmit pokeweed mosaic to the 
cucumber. All of the inoculation experiments mentioned above 
were made by artificial methods, using the expressed juices or crushed 
tissues of mosaic plants. 
The possibility of a relationship between the mosaic disease of 
pokeweed and that occurring on the cucurbits was first indicated by 
observations made by W. W. Gilbert at Holland, Mich., in 1915, 
where mosaic pokeweeds were found in abundance near a field of 
mosaic cucumbers. The first successful cross inoculations from the 
pokeweed to the cucumber were obtained in the course of experiments 
with the cucumber aphis. 4 Aphids from mosaic cucumber plants 
had been colonized on certain plants of species supposedly insuscep¬ 
tible to cucurbit mosaic, in order to determine whether the infective 
principle of the disease was transmissible to the offspring of aphids 
which had fed on mosaic plants. Among the plants used were a few 
pokeweeds, and it was noted that the majority of these plants showed 
symptoms of a mosaic disease within two weeks after the aphids had 
been placed on them. As a result of this evidence, further cross¬ 
inoculation experiments were undertaken, in which aphids were used 
as a means of inoculation. 
METHODS OP INOCULATION 
As already indicated by the experiments with the milkweed, the use 
of aphids as a means oi inoculation has been the most important 
factor in demonstrating the wide host range of the disease. In the 
earlier studies of cucurbit mosaic (8) it was found that inoculations in 
which aphids were transferred from mosaic to healthy cucumber 
plants resulted in nearly 100 per cent infection, but that a consid¬ 
erably lower percentage of infection was obtained when plants were 
artificially inoculated with the crushed tissues or expressed juices of 
mosaic plants. The advantages of insects as a means of inoculation 
became more apparent in the cross-inoculation experiments described 
in this paper. 
In the case of the milkweed, pokeweed, and martynia, earlier cross 
inoculation by artificial methods (8) had given only negative results, 
and it was supposed that these plants were not susceptible to cucurbit 
mosaic. The use of aphids as a means of inoculation, however, 
demonstrated later that cucurbit mosaic could be transmitted to all 
of these hosts and to several other plants of widely separated families. 
While infection has since been produced on many of these hosts by 
artificial methods of inoculation, the first infection has always 
resulted from insect inoculations and the results have been much 
more consistent than those obtained by the artificial method. 
4 The studies of the mosaic disease of pokeweed were conducted by the junior writer of this paper and 
presented as a master’s thesis at the University of Wisconsin. 
59836—25f-3 
