23 



naturally to a standstill. The recommendation of every regulation for the 

 prevention of infection by the removal of infected potatoes from the field, or 

 by deep subsoil cultivation, or the burning of diseased straw in grain epi- 

 demics, we consider to be a work with insignificant results as contrasted with 

 the effect of changed life conditions for the parasite. The amount of in- 

 fected material in extensive districts does not come under consideration at 

 all. At times in the case of damp rot, soil bacteria co-operate and form a 

 dense condition of the soil. If atmospheric influences make themselves so 

 felt in certain soils that certain bacterial groups are able to attack potatoes 

 or other fruits of the field, the number of the causative agents of the disease 

 originally present is almost of no significance. 



The last named examples of parasitic epidemics due to such micro- 

 organisms as may be assumed to be constantly present in the soil or the air, 

 make clear to us, however, how little prospect of success is offered for com- 

 batting an epidemic once it has broken out. A greater protection for our 

 cultivated plants lies in preventive methods. Such a preventive process in 

 epidemics, aside from the formation of an universal plant hygiene, can, how- 

 ever, be induced by the drawing up of a chart of pestilences; that is, a sum- 

 mary of plague centres for each individual epidemic. In the correspondence 

 of certain characteristics for a number of plague centres, single factors are 

 especially distinguished as fundamental for the production of an epidemic ; 

 for example, dryness in light soils is shown to be favorable for fly epi- 

 demics of grain or for the heart-rot of sugar beets etc. Having thus deter- 

 mined weather and soil combinations dangerous for each individual epidemic 

 one can make one's attack prophylactically by means of cultural regulations 

 as soon as the threatening combination of conditions continues for some time. 

 Direct means which kill the parasites, such as sprinkling with copper sulfate 

 or dusting with sulfur, will then act only as hinderanccs to the epidemics if 

 used preventively. 



6. Artificial Immunization and Internal Therapy. 



It is quite natural that in phytopathology the same course of ideas has 

 developed as in animal pathology and accordingly it is not strange that there 

 has gradually become evident a theory of immunizing plants artificially ; i. e., 

 of so changing their bodily composition that the parasites will no longer find 

 the nutritive soil necessary for colonization, for their wider distribution. 



There already exist several works along this line in which, following in 

 part serum therapy, use is made of immunifying substances obtained from 

 the parasite itself, and again where mineral salts are used. Along the former 

 line belong Beauverie's^ investigations with Botrytu cinerea and those 

 of Ray- with very different kinds of parasites. The latter obtained 



1 Beauverie, J., Essai d'immuni.sation des vegetaux centre les maladies crypto- 

 gamiques. Compt. rend. Paris 1901. 11, p. 107. 



2 Ray, J., Cultures et formes attenuees des maladies cryptogamiques. Compt. 

 rend. Paris 1901. II, p. 307. 



