Causes of Diseases in Crops 89 



of insects as carriers and disseminators of plant dis- 

 eases. We are becoming increasingly aware of the 

 r61e which insects play in the carrying and dissem- 

 inating of human and animal diseases. They are 

 equally resjjbnsible in distributing plant diseases, 

 acting as carriers of spores of parasitic fungi which 

 may adhere to any part of their body. Insects both 

 by feeding on plants or in searching for the nectar of 

 the blossoms are likely to come in contact with dis- 

 eased parts of plants. In this way their bodies may 

 become coated with parasitic bacteria or spores of 

 fungi, which are thus carried from plant to plant and 

 from field to field. The striped cucumber beetle, 

 for instance, is known to carry and to spread about 

 the virus of cucumber mosaic, and the germ of 

 cucumber wilt {Bacillus tracheiphilus) . Likewise, the 

 Colorado potato beetle is a carrier of the germ of the 

 Southern blight {Bacillus solanaceafum) of tomato 

 and potato. Noxious insects act not only as direct 

 carriers of spores of parasitic fungi and bacteria, but 

 also induce diseases through the bites and wounds 

 which they inflict on plahts when feeding. It is there- 

 fore very essential that every effort which aims at con- 

 trolling fungous pests should also take in consideration 

 the control 6f noxious insects; see page 367. 



E. Diseases Induced by Parasitic Flowering 

 Plants 



Fungi and bacteria, as we have seen, are low forms 

 of plant life. These derive their food either from 



