306 
THE PARASITIC DISEASES OF PLANTS. 
the plant, between the stem and the root. The parasite that 
causes it has the power of growing upon a large series of plants, 
producing tumors in various parts of the plant which injure it 
more or less, according to the extent of the infection. Among the 
plants that may be infected with it are the raspberry (Fig. 55), the 
daisy, the hop, the radish, the cabbage, the tobacco, the sugar beet, 
the grape, the tomato, the oleander, the apple, and some others. 
It is unusual for a parasite to have such a long list of possible hosts, 
but in all these plants it has been demon- 
strated by Smith that tubercles will be 
produced by the inoculation of pure 
cultures of the organism. It is the cause 
of considerable losses to horticulturalists. 
Root Tubercles of Legumes. These 
have been considered in a different con- 
nection (Chapter VII), but they are 
properly classed here. They are cer- 
tainly caused by parasitic bacteria, 
although in this case apparently both the 
parasite and the host are benefited by the 
association, a condition sometimes called 
symbiosis rather than parasitism. 
Remedies. Remedies for the bacterial 
diseases are not as yet very satisfactory. 
Spraying, so frequently efficient against 
fungoid diseases, is of no value here, be- 
cause the bacteria are always within the tissues of the plant where 
the spray cannot touch them. Hence in dealing with plant diseases 
in general it is always desirable to know whether they are fungoid 
or bacterial, since in the latter case spraying is always useless. 
Each disease has to be met by devices adapted to the peculiar 
nature of the disease, and no general principles can be given beyond 
that already pointed out on page 295. 
FIG. 55. The crown gall on the 
root of the raspberry. 
