2 Minnesota Plant Diseases. 



of a part of a plant or of the whole plant is threatened, we rec- 

 ognize disease in that plant. The change in favorable condi- 

 tions may be so slight that the shortening of the life of the plant 

 or its parts is not apparent. One does not recognize disease 

 in such a case, although it is essentially similar to that of well- 

 recognized diseases. There are, then, between health and dis- 

 ease in plants imperceptible gradations and no sharp lines of 

 demarcation. A farmer who intelligently strives for all of the 

 most favorable conditions of his crop is in reality combating 

 disease. A great loss to agriculture annually occurs which does 

 not usually pass for disease. When the grower of plants real- 

 izes this, and when he joins his efforts with those who are seek- 

 ing methods of combating diseases, then more rapid strides will 

 be possible in methods of investigation and prevention. The 

 more knowledge a farmer possesses of the conditions favorable 

 and unfavorable to the numerous diseases which affect his crop, 

 just so much more successful will he be in his efforts toward 

 preventing disease. Agriculture really resolves itself into one 

 great problem, the prevention of plant disease in the broader 

 sense. 



There are three factors to be considered in a plant disease: 

 (i) The immediate cause of a disease, e. g., fungi, bacteria, 

 insects, etc., as enumerated above. 



(2) The immediate effect in the anatomy, form and physi- 

 ology of the host plant and the effect in inheritance. 



(3) The previous condition and disposition of plants which 

 may seriously affect the susceptibility of those plants to a cer- 

 tain disease ; in other words, the predisposition of plants toward 

 disease. 



It is therefore apparent that one must study not only the 

 immediate cause of a disease but the predisposition or immu- 

 nity of plants toward that disease. An appreciation of these 

 principles finds expression in the selection of varieties for spe- 

 cific purposes and in the more detailed study of the life-histories 

 of diseases. By such study an exact knowledge of the habits 

 of a parasite are obtained and it is only by means of this knowl- 

 edge that we can intelligently devise methods of prevention. 

 Too much stress cannot be placed on the necessity for accurate 

 work in the observations of the habits of a parasite and of its 



