PLANT DISEASES 



585 



[Chap. XLV 



parts of the world. The tobacco mosaic has been shown to be caused 

 by a protein of high molecular weight, which may be purified by re- 

 peated recrystalhzation without losing its property of causing the dis- 

 ease. This causal agent increases in number in contact with the proto- 

 plasm of the host plant. It cannot be cultivated in non-living culture 

 media. 



Fig. 266. Symptoms of mosaic disease on leaves of tomato and bean. Photos: A, 

 by J. D. Wilson; B, by U. S. Department of Agriculture. 



Plants seldom recover from virus infections. The plants may survive 

 infection for long periods of time, but the virus causes decreased 

 photosynthesis, decreased yields of crop plants, small and distorted root 

 systems, and increased susceptibility to other parasites. Viruses are 

 usually not localized in certain tissues and organs, but permeate all living 

 parts of the plant. Herbaceous plants are apparently more susceptible 

 to viruses than woody species, although lethal virus diseases of elm, 

 plum, and peach are well known. Vimses are transmitted from dis- 

 eased to healthy plants by insects, such as leaf hoppers, aphids, and flea 

 beetles, and sometimes on the tools used in cultivating, pruning, and 

 grafting. Certain viruses are transmitted through the seed, while others 

 are not. Some viruses may be carried over from one season to the next 

 in other hosts such as weeds and wild plants which grow nearby. Con- 

 trol has been accomplished by the removal of such additional hosts. 



Other control measures include the eradication of diseased plants, 

 control of the insects that transmit viruses, use of disease-free seeds, 

 the avoidance of potentially infected parts of plants in grafting and 

 vegetative propagation, and the selection of less susceptible varieties. 



