Minnesota Plant Diseases. 



93 



particular towards that disease. The first factor will be con- 

 sidered later. The second has been discussed in Chapter VI. 

 We will now examine mo-re in detail the third factor, i. e., pecul- 

 iarities in internal and external conditions which make a plant 

 more or less susceptible to disease. 



Predisposition. This susceptibility is in almost all cases a 

 specific one toward a certain disease and less often toward many 

 diseases. When a plant is subject to the attack of numerous 

 agencies w r e can easily imagine some change in conditions, 

 as, for instance, transplanting from a dry to a moist atmos- 

 phere, which would favor the attack of all of these diseases. 

 The conditions become such that fungus attacks in gen- 

 eral are facilitated. But most predispositions are in the nature 

 of special conditions which are favorable to only one spe- 

 cial disease or class of diseases. This distinction is expressed 

 in the terms general and special predisposition. An illus- 

 tration may make this clear. Wheat rusts are of different 

 kinds caused by several fungi. In general, moist warm weather 

 in the growing season predisposes all kinds of wheat plants in 

 many ways to the attack of rusts, and such conditions furnish 

 general predisposition. But if a certain variety of wheat be 

 particularly susceptible to a given rust fungus, abundant in the 

 region into which the wheat is introduced, the new condition of 

 position in the wheat plant predisposes it very much to that 

 disease. Other varieties of wheat, less susceptible to that par- 

 ticular disease, might be unaffected so that we may have here a 

 special predisposition. 



It is a fact which must not be lost sight of, that the predis- 

 position of the plant in itself may not be harmful to that plant, 

 but may be a condition which might be highly recommended 

 when considered alone. But it is the other factor, the fungus or 

 insect, which may be the disturbing influence and which is 

 especially favored by this condition of the host plant. Such a 

 distinction is of very great importance to practical agricultur- 

 ists and horticulturists because it is not only the immediate 

 condition of the plant or, on the other hand, the presence of 

 disease-causing conditions, but it is the relationship between 

 these two factors that is all important. There is another fact of 

 great importance that must be emphasized. No plant, as far as 



