PLANT DISEASES 



279 



the blight are blown on to the leaf, they may fall be- 

 tween these spots or into them. The spores, it will be 

 remembered, do not grow unless water is present, when 

 they break up into swimming spores. When water is 

 present, the Bordeaux mixture spots, which may have 

 dried out in dry weather, again 

 diffuse through the drop and 

 the little swimming spores find 

 themselves in a little lake of 

 poisoned water. The finer the 

 spray, the more numerous are 

 these little lakes and the less 

 numerous are the safe places 

 between them. It will also 

 be seen from this that spray- 

 ing is a preventive measure, 

 not a cure. It would do no 

 good t spray a plant after it 

 already has the blight, because 

 the blight in such a plant is 

 inside of the leaf. More- 

 over, when heavy rains wash 

 off the Bordeaux mixture, it must again be applied in 

 order to keep the plants covered by the mixture. When 

 the Bordeaux is properly applied, it will not injure the 

 foliage. 



There are large numbers of plant diseases which 

 attack the leaves and above-ground portions of plants 

 which can be prevented by spraying. The most com- 

 monly used material for spraying is the Bordeaux 

 mixture, though other substances are sometimes used. 

 The life stories of the diseases which can be prevented 

 by spraying are not always similar to that of the blight 



FIG. 144. A Barrel Pump for 

 Spraying Plants. 



