1908.] PUBLIC DOCUMENT— No. 33. 149 



reduce the power of resistance, consequently the organism falls 

 a prey to the ever-present germ. 



The causes underlying susceptibility are much better under- 

 stood than those of immunity. Why it is that the moment a 

 plant becomes weakened various organisms attack it, is not 

 fully understood. We have observed many instances of cer- 

 tain treatments weakening plants, and as a result it is surprising 

 to note the number of organisms which always attack the plant 

 a very short time afterwards. The changes which actually take 

 place in an organism in a depressed condition are not known, 

 but many of these may be of an abnormal chemical nature. It 

 is possible that these abnormal chemical changes stimulate 

 organisms to attack weakened plants ; that is, the loss of im- 

 munity increases the susceptibility of the organism to disease, 

 due to vital depressions in the plant, which may result in the 

 giving off of substances that act as a stimulus and attraction to 

 invading organisms. Briefly stated, susceptibility to disease 

 may be associated with chemotactic irritability. 



Some crops are probably rendered more susceptible to fun- 

 gous diseases by cultivation. The limitations of forcing have 

 undoubtedly been overstepped in some cases, and this is espe- 

 cially true of the carnation, which has been much troubled with 

 the wet and dry stem rots since the modern methods of forcing 

 have come into vogue. 



In the case of outdoor crops, great differences exist in the 

 environment, due to climatic influences. The conditions may 

 be such that a disease constantly causes loss in one locality and 

 scarcely any in another ; and, while it may be necessary to 

 spray for a trouble in one State, in others no attention need 

 be given it. No doubt in some instances it would lie wiser to 

 devote one's energies to cultivation, as a means of preventing 

 plant diseases, than to resort to the use of fungicides. Our 

 most skilled agriculturists, such as florists and market gar- 

 deners, seldom if ever resort to spraying, and in greenhouse 

 culture the use of fungicides is practically unknown. Certain 

 crops are greatly benefited by being sprayed with fungicides ; 

 but, on the other hand, there are crops which have been 

 sprayed for many years with little or no benefit as far as the 

 control of pathogenic fungi is concerned, and the money spent 



