152 DISEASES OF GLASSHOUSE PLANTS 
particular strain of mosaic. disease to be combated. 
Neighbouring weeds as well as cultivated plants must be 
taken into consideration. Growers should make a careful 
examination of weeds and other plants growing in the 
immediate vicinity of the nurseries, and any plants bear- 
ing symptoms suggestive of mosaic disease should be dug 
up and burned. No suspected plant should be given the 
benefit of the doubt. In relation to tomato mosaic it is 
now known that certain solanaceous weeds like the 
black nightshade (Solanum nigrum) and the bittersweet 
(Solanum dulcamara) are susceptible hosts, and should 
be eliminated from the neighbourhood of tomato 
nurseries, as they may act as centres for the spread of 
the disease. Similar dangers exist in the petunia and 
potato, and manure harbouring potato tubers should not 
be used, as, if these contain the virus of mosaic disease, 
infection may spread from the diseased shoots when 
these develop. . 
Alternative hosts for cucumber mosaic in this country 
have not been discovered, and it is somewhat difficult to 
understand from whence the infection comes, unless one 
accepts the statement which observation on commercial 
nurseries seems to support—that the disease is trans- 
mitted in the seed. There are no indications that tomato 
mosaic is carried in the same way. 
In the case of sporadic infection of plants in a nursery 
during the early days of the season, care should be taken 
to remove immediately the affected plants before the 
disease can spread. In the case of mid-season infection, 
no useful purpose would appear to be served by removing 
infected plants. The difficulty which presents itself 
when an attempt is made to effect a control by eliminating 
centres of infection is that these are not always easy to 
identify when the symptoms are slight, and in the case 
of “carrier” plants showing no outward signs of the 
disease identification by simple means is impossible. 
The Determination and Elimination of Agents by which 
the Disease is Spread.—The chief means by which the 
