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. 



