PREVENTION OR CURE OF PLANT DISEASES 19 



duction, followed by experimenting, testing, and improving. 

 Such investigation is ever proceeding with increasing 

 activity, and yearly new or improved methods of combat 

 are devised, so that the future may with confidence be 

 relied upon to see the subjugation of many of the foes that 

 are as yet unconquered. 



Methods of Prevention 



All methods of prevention aim at the destruction of 

 the causal organism, the diminution of its numbers, or the 

 strengthening of the crop plant so as to withstand its attack. 

 The various methods employed maj'', for convenience of 

 reference, be here summarized as follows : — - 



1. Killing the causal organism as it rests upon the seed. 

 — Examples : oat or wheat smut, cabbage black rot. 

 For this purpose the seeds (sometimes cuttings or even 

 whole plants are similarly treated) are subjected to the 

 action of disinfecting chemicals long enough to kill the 

 organism causing the disease, but not long enough to kill 

 the seeds. Chief among such disinfectants are: — 



A. Formalin (40 per cent formaldehyde). 



B. Corrosive sublimate. 



C. Copper sulphate, followed by weak limewater. 



D. Formalin gas generated in a gas-tight disinfecting 

 room. 



2. Killing superficial fungi by applications upon the 

 green parts of the plant. — Examples : powdery mildew 

 upon grape, gooseberry, or rose. This method is apphcable 

 only with the purely superficial fungi, since internal pari- 

 sites cannot be so reached without injury to the host plant 



