Chapter Fifteen 

 INSECTS AND DISEASES 



Every plant has its disease and insect enemies. In fact 

 so numerous are these pests that amateur gardeners give up 

 the struggle because of the study, effort, and expense in- 

 volved in their control. The gardener's despair is under- 

 standable in view of the suggestion of the eminent plant 

 pathologist, Dr. Smith, that "Plants may be found to have 

 as many kinds of diseases as the human race." Dr. L. O. 

 Howard, an entomologist of note, is even more gloomy in 

 saying that by destroying man's food supply "Insects might 

 destroy man." The annual loss of plants from diseases and 

 insect pests can only be estimated but it reaches into billions 

 of bushels of foods. The money loss from insects alone is 

 estimated at more than three billion dollars. To the actual 

 money loss because of the damage of pests must be added 

 the cost of efforts toward the restriction of such damage, for 

 example, 180,000 tons of poisonous bait was used in 1938 to 

 control grasshoppers in the United States. 



Successful control of insects and diseases depends on pre- 

 vention rather than cure. A covering of poison spray on 

 leaves not only protects the leaves from damage by killing 

 destructive insects by the thousand but also prevents their 

 reproduction. As is the case among humans, insects often 

 act as agents to spread diseases among plants. 



It is even more important to prevent plant diseases be- 

 cause when the insects are destroyed the plant they have 

 injured may recover, but when a plant disease enters the 

 plant it spreads rapidly, feeding on the protoplasm and solu- 

 ble foods of the plant, and cannot be touched with a spray 

 solution. Since it is almost impossible to kill a disease in 



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