THE CAUSES OF DISEASES OF PLANTS 227 



Killing fungi with poisons. Fortunately the spores of 

 most kinds of plant diseases will not grow in contact with 

 certain substances. Two of the best of these chemicals 

 used to destroy fungus spores are bluestone (copper sul- 

 fate) and formalin. The buds, the fruit, or the leaves 

 are sprayed with a mixture containing bluestone. If this 

 is done before the fungus plant has passed through the 

 skin of the fruit, the crop is often saved. Seeds of oats 

 from a smutty crop are dampened with formalin to kill the 

 spores on the seed. The white mildew on the leaves of 

 the rose is easily killed by sprinkling on it a solution of 

 one ounce of liver of sulfur to two gallons of water. 



Prevention of plant diseases easier than cure. In most 

 cases, however, it is useless to try to cure plant diseases 

 by treatment after the fungi have entered the green plant, 

 where no poisons can reach them. Spraying fruit trees is 

 done to prevent, and not to cure, diseases. The poison 

 generally used to ward off diseases of fruit trees, Irish 

 potatoes, and others is Bordeaux mixture, which contains 

 bluestone (see Appendix). 



Weak plants become diseased. Plants that are thrifty 

 and well nourished are less apt to catch certain plant 

 diseases than those that are weak, starved, or unwisely 

 fertilized. There are many diseases of plants that are not 

 due to germs. These are generally due to poor drainage 

 or other unfavorable conditions of soil or climate. Such 

 diseases are not contagious. 



Fungous diseases spread rapidly because of the light 

 spores, which are blown or carried great distances. 

 Nearly all the peaches in an orchard may be ruined by rot 



