14 Diseases of Economic Plants 



Methods of Prevention 



All methods of prevention of plant diseases due to par- 

 asites 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 may, for convenience, be summarized as follows: — 



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

 — Examples: oat or wheat smut, cal)bagc black-rot. For 

 this purpose the seeds (sometimes cuttings or even whole 

 plants are similarly treated) are subjected to the action of 

 heat or disinfecting chemicals long enough to kill the causal 

 organism, but not long enough to kill the seeds. Chief 

 among such disinfectants are : — 



Formalin (40 per cent formaldehyde). 



Corrosive sublimate. 



Copper sulfate, followed by weak limewater. 



Heat. 



2. Killing superficial fungi by applications upon the 

 green parts of the plant. — Examples: powdery-mildew 

 upon grape, gooseljerry, or rose. This method is applicable 

 only with the purely superficial fungi, since internal para- 

 sites cannot be so reached without injury to the host plant 

 itself. It finds prominent application with the powdery- 

 mildews. The chief applications used are : — 



Liver of sulfur. 

 Bordeaux mixture. 

 Ammoniacal copper carbonate. 

 Sulfur, or sulfur and lead arsenate. 

 Lime-sulfur. 



3. Killing superficial fungi hibernating upon the bark of 

 the dormant plant or plant supports. — Examples: apple 

 hypochnose, peach curl. For this purpose cleansing sprays, 

 often called dormant sprays, may be used. Since the parts 

 of the host receiving the spray are highly resistant to their 

 poisonous action, the sprays thus employed may be much 



