INSECTS AND DISEASES 57 



are much more difficult to control, because once the 

 disease organism is within the tissues of the host 

 plant nothing is seen until the fruiting stage of the 

 disease appears on the surface. Treatment must 

 consist in killing the spores of the organism before 

 they germinate and enter the plant. Afterward any 

 treatment that would reach the disease-causing or- 

 ganism would likewise destroy the tissues of the 

 host plant. It will be seen at once that spraying,, 

 for diseases especially, must be preventive rather than 

 curative in its nature. No amount of treatment 

 can ever cure a rotten cabbage or tomato. Proper 

 treatment at the right time may prevent the trouble 

 entirely. 



The three groups of diseases mentioned above 

 belong to the larger group of low plant life known 

 as fungi. These are simply microscopic plants 

 which grow and draw their nourishment from other 

 plants rather than directly from the soil and air. 



Still a fourth group of disease organisms deserves 

 mention, namely, bacterial diseases. While the 

 number of enemies found in this group is not so 

 large as in the fungous group, they are far more 

 difficult to control, and include many of the most 

 destructive and persistent diseases troubling the 

 gardener. The dreaded "damping-off" disease of the 

 seed and cutting beds, the wilt of the lettuce and 

 cabbage fields, as well as a number of other serious 

 pests, belong to this group. Spraying does not 

 seem to be effective in their control. They are bac- 

 terial in their character and spread rapidly through 

 the tissues, becoming particularly destructive when 

 climatic conditions become favorable for their de- 

 velopment. 



Many of the bacterial diseases, together with some 



