Plant Diseases 



435 



ment and the subsequent drying of the grain are so difficult to 

 perform that this method of prevention is rarely used for the 

 general crop. It is used, however, for treating grain that is to 

 be planted for the production of seed for the following year. 



Galls. Among the most striking examples of abnormal develop- 

 ment of tissues and organs are the galls produced on a great 

 variety of plants by insects, and more rarely by fungi and bac- 

 teria. Almost every one has seen the large papery galls of oak 

 leaves, the velvety gall of the rose, the cone-like shoots of the 

 pussy willow, and the swellings in the stems of goldenrod. These 

 are all brought about in some unknown way by insects living in 

 the plant tissues. 



Downy mildew. A downy mildew commonly occurs on leaves 

 and stems of grape leaves and may cause a reduction of the grape 

 crop by injuring the leaves and causing them to drop. In some 

 cases the fungus may attack the green fruit, causing it to wither 

 and drop to the ground. 



Brown rot of stone fruits. One of the most destructive dis- 

 eases of cherries, plums, and peaches is the brown rot. The 

 fungus causing this disease is carried over the winter on the 

 mummied fruits hanging on the trees or lying on the ground be- 

 neath. Beginning in June, spores are carried to the developing 

 fruits where they germinate, and decay follows the growth of 

 hyphae, resulting in brown spots and finally the withering of 

 the entire fruit. When infection occurs late, at the time of 

 gathering, this disease may cause serious losses during the mar- 

 keting of the fruit. 



Brown rot may be controlled by carefully removing all mum- 

 mied fruits at the end of the year, and by spraying with a mixture 

 of lime and sulfur at proper intervals during the season. 



Mosaic disease. This disease has been especially injurious 

 to tobacco, but it also affects tomatoes and to a less extent a 

 great variety of wild and cultivated herbs. The external signs 

 are a light-green mottling of the leaves or distorted and stringy 



