THE MOSAIC DISEASE OF CUCURBITS. 59 
field until July 19, although all plats were carefully inspected at 
intervals of two to four days. On July 19 two plats out of six 
within a radius of a mile developed a few cases of mosaic, and six 
days later 25 to 30 per cent of the plants in both plats showed 
symptoms of the disease. The remaining fields also showed from 
5 to 15 per cent of mosaic plants on that date. All cases were of 
the same age and seemed to have developed almost simultaneously. 
In view of the fact that insects had been attacking the plants for 
four weeks previous to the appearance of the disease, while the 
period of incubation is normally 7 to 12 days, it is difficult to explain 
such a sudden and extensive outbreak of the disease on the basis of 
insect overwintering and transmission. 
That seedling plants may be infected when small but show no 
signs of the disease until they reach a certain period in their devel- 
opment has been considered a possible means of connecting such 
sudden outbreaks of the disease with insect agencies. No such 
abnormally long incubation period has ever been noted, although 
at least 200 seedlings have been kept under observation for several 
" weeks after inoculation. Secondary inoculations from such plants 
have never produced the disease except in cases where there were 
definite symptoms of mosaic in the original plant. There is no 
evidence, therefore, that such a prolonged incubation period does 
occur. 
Further evidence against the theory that insects carry the disease 
over winter is also furnished by the results of the work on disease 
transmission during 1917. As shown in Table XVIII, the beetles 
collected after frost had killed the vines failed to induce the disease 
when placed in cages with healthy plants, while all beetles collected 
earlier in the season contained some individuals which were carriers 
of mosaic. It is quite probable, therefore, that only a very small 
percentage, if any, of the beetles which go into hibernation carry 
the disease, since they are present in the fields for some time after 
all mosaic vines have been killed by frost. In view of the results 
obtained in the artificial hibernation studies, it is likely that only a 
few of those beetles which go into hibernation survive the winter, 
and field observations also indicate that only a small percentage of 
the insects present in the fields in the fall appear in the spring. If 
this is the case, a large number of insects would necessarily have to 
be tested to insure finding individuals which carry the disease, if 
ever any such are present. Rand (23) has found that only a small 
number of the striped beetles which survive the winter are carriers 
of Bacillus tracheipJiilus, and this is even more likely to be the case 
with cucumber mosaic. The number of beetles so far tested is too 
few, perhaps, to warrant definite conclusions, but the evidence to 
date does not lend much support to the theory that insects are 
instrumental in carrying the disease over winter. 
