214 DISEASES OF PLANTS 



sects which might otherwise infect and infest the larger 

 plant. The application of dusts by blowing on foliage and 

 bark is designed to interfere mechanically as well as chem- 

 ically with the invasion of host by parasite. Sprays 

 differ from dusts mainly in the manner of application, for 

 though they also are blown upon the foliage and bark, 

 they are suspensions or solutions in water, and the poison- 

 ous material left by evaporation acts both mechanically 

 and chemically upon the invading organisms. 



The Medical Treatment of Disease with the object 

 of curing also consists in the application of sprays and 

 dusts designed to offer mechanical inconvenience to para- 

 sites or to exert a poisonous influence upon them. Thus 

 Bordeaux mixture, a spray, may serve not only to prevent 

 infections but may also be used in the treatment of mil- 

 dews or other fungi. Sprayed upon the leaves of roses, 

 vines, etc., attacks of mildew may be cured entirely by 

 this means. 



The Surgical Treatment of Disease. — In case of 

 invasion below the surface of the host, surgical treatment 

 must commonly be resorted to in plants, and in the 

 more serious cases amputation cannot be avoided. 

 In order to prevent the spread of infection from 

 one part to another, the same precautions must be 

 employed in sterilizing or destroying all infected tissues 

 and in sterilizing the instruments used in operation. In 

 fact, with the modifications required by the differences in 

 the organisms, the same principles of hygiene control 

 the treatment of plants as of animals. It is thus evident 

 that the practice of " tree surgery " involves great care in 

 the treatment of living tissue and in the handling of in- 

 struments (sterilization). Where care and skill are lack- 

 ing the result can entail only pecuniary and other loss. 



Sx^mmary. — The causes and treatment of disease are 

 essentially similar in plants and animals. Loss from un- 

 favorable weather or other environmental conditions, lack 

 of proper coordination in the vital processes of the organ- 



