MOSAIC DISEASES 153 
disease is spread from plant to plant are by insects and 
by the workers. 
The complete elimination of insect carriers is almost 
impossible, but much can be done by reducing the number 
to a minimum. The white fly pest of tomato houses 
may be effectively controlled by hydrocyanic acid gas or 
tetrachlorethane, while aphides, when present, are easily 
controllable by many simple insecticidal fumigants. 
By careful education of the workers, the rate of spread 
of the disease by this means can be reduced. They 
should be made to understand that merely crushing a 
diseased leaf with their fingers and then bruising a 
healthy plant, thus conveying infected juices to it, is 
sufficient to transmit the disease. It is imperative in 
cultural operations that all diseased plants should be left 
alone until the healthy plants have been treated, and in 
no case should a healthy plant be touched immediately 
after a diseased one. It is claimed that infection is 
spread by the clothes rubbing against a diseased plant 
and then a healthy one, but this does not appear to 
have been confirmed experimentally. After handling a 
diseased plant it has been shown that the infection can 
be removed from the hands by washing with soap and 
water. Pruning knives, after contact with a diseased 
plant, should be wiped on a rag moistened with some 
disinfectant like lysol. 
The Determination of Cultural Conditions necessary to 
Increase the Resistance of Susceptible Plants.—As yet but 
little information in this respect is available, and investi- 
gations should yield important results. 
The Breeding of Immune Varieties——Many attempts 
have been made to breed varieties immune to this 
disease, but while satisfactory results have not been 
obtained, there are indications that success will eventually 
be forthcoming. In the case of cucumber mosaic the 
variety Butcher’s Disease Resister is highly resistant. 
