154 REPORT OF STATE BOARD OF HORTICULTURE. 



cide was not properly made, that the spraying was not done 

 early enough in the season, or that the applications were not 

 thorough or persistent enough. If we wait until we see the 

 disease at work before we begin spraying our efforts will not 

 result in success, for the reason that when we see the disease 

 it is certain that the fungus spores have germinated and the 

 fungus has grown into the affected part of the host-plant. In 

 such cases it is impossible to destroy the fungus without de- 

 stroying the diseased part of the host. The most that can be 

 hoped for in such cases is that the disease may be prevented 

 from spreading to the healthy plants or plant parts. If the 

 spraying is not thorough, so that all parts of the host are cov- 

 ered, spores may fall upon the unprotected parts and grow 

 as readily as if no fungicide had been used. Or if the appli- 

 cations are not frequent enough, so that the fungicide is 

 washed off, or new plant parts are developed and left un- 

 sprayed, attacks of fungi may take place as readily as if no 

 spraying had been done. It should be remembered that no 

 fungicide will restore any plant part once destroyed or injured, 

 hence the necessity of preventing attacks of fungi, and this 

 can be done by an early, thorough, and persistent use of 

 fungicides. 



WILL SPRAYING PREVENT ALL PLANT DISEASES? 



Several plant diseases, of which " peach yellows " is an ex- 

 ample, are not, so far as known, produced by organisms, and 

 these diseases can be neither prevented nor controlled by 

 fungicides. 



Other plant diseases are produced by bacteria that live in the 

 tissues of diseased plants. These minute organisms seldom 

 appear on the surface of the host-plant and consequently 

 would not usually be reached by spraying. Such a disease is 

 the pear blight. It is often the case that a disease attacks 

 only the underground portion of the plant. It is clear that 

 a disease of this nature could not be prevented or controlled 

 by spraying. Potato scab is an example of diseases of this 

 kind. In short, it is only those fungous diseases that origin- 

 ate from spores on the aboveground portions of plants that 

 may be prevented by spraying, 



WILL IT PAY TO SPRAY? 



Whether it will or will not pay to spray must depend upon 

 circumstances. It is of prime importance to know whether 



