INTRODUCTION. 16 



But vegetable therapeutics will often yield imperfect results in spite of 

 all the attention brought to bear in the application of appropriate 

 remedies, for it is difficult to dislodge or to destroy in the interior 

 and on the exterior of a plant without injuring it the parasites which 

 develop there, surrounded by the very efficient means of protection 

 which nature has given them ; and if we insist on this axiom, that a 

 plant disease cannot be cured, but that it can only be diminished or 

 its extension prevented, the important role which preventive methods 

 play in the struggle against plant diseases will be understood. 



Prophylaxy. — Prophylaxy is that part of medicine which deals with 

 the means of guaranteeing against disease and preventing it. Knowing 

 the cause or the causes of the diseases it is possible to protect plants 

 efficiently against them. The knowledge acquired as to the reactions 

 of the organism, and the means by which it naturally arranges to 

 defend itself against disease, have enabled prophylaxy to utilize 

 physiological processes instead of agents destructive to parasites. It 

 is necessary to differentiate between therapeutic prophylaxy and 

 hygienic prophylaxy. The former utilizes therapeutic agents, surgical 

 processes as well as antiseptic insecticides, fungicides. The latter 

 employs dietetics, stimulants of growth, rational feeding, selection of 

 vigorous and hardy species. Medicine in its application to plants is 

 in fact as complicated as when applied to man, and it is not surprising 

 to see it necessary to take at the same time prophylactic and thera- 

 peutic measures in order to have crops free from disease. 



Therapeutic Prophylaxy. — When the cause of a disease is known, 

 its evolution and that of the parasites which produce it, it is com- 

 paratively easy to find the means of checking it by preventive 

 measures. These treatments may be very often carried out at a time 

 when the plant can bear them with impunity ; in winter when the 

 delicate organs have disappeared and when the sap is at rest. One 

 must never wait until a disease manifests itself, even if the possibility 

 of its appearance is not absolutely certain. Preventive treatments if 

 they are not always capable of removing all the effective and adjuvant 

 causes of disease will minimize them. When the cause is a parasitic 

 one, the object pursued is not to destroy all the parasitic elements, but 

 to reduce them to their normal or natural number increased by our 

 methods of cropping. In these conditions, parasites having always ex- 

 isted and their complete destruction being as chimerical and as useless 

 as a complete disinfection of the air which we breathe, with the object 

 of destroying all microbes, disease is no longer to be feared, because it 

 no longer causes us appreciable injuries. 



Preventive Surgical Treatments. — Operatory medicine may be 

 of great assistance in the prevention of plant diseases ; in fact the 

 suppression of everything which may transmit a disease from one year 

 to another is often capable of giving radical results — excision of the 

 diseased parts, removal of branches attacked or bearing spores or eggs, 

 washing of the bark of the trunk and branches to suppress refuges 

 formed by mosses and lichens for acari, aphides, and coleoptera. In- 

 tervention by naked hand plays a role not less important, by the 

 eollection and suppression of the old organs of plants, leaves, and 



