PLANT DISEASES 171 



as the common powdery mildew of the lilacs, the powdery 

 mildew of roses, and the mildews on gooseberry and cherry 

 foliage. 



The other type of mildew is called downy mildew. This 

 type of disease is caused by a fungus which grows within 

 the tissues of the host rather than on the surface. It is illus- 

 trated by such diseases as the downy mildew of the lettuce 

 which is very common in greenhouses, the downy mildew of 

 onions, and the downy mildew of grapes. 



Spot diseases. Many diseases are characterized by the 

 formation of definite disease spots on the foliage, fruit, or other 

 parts of the host plant. In cases of severe attack these spots 

 may become so numerous as to lead to the death of the host, 

 while in other cases the effect is not so severe. 



The leaf-spot diseases of beet, cherry, strawberry and to- 

 mato are good examples of this class of diseases as they occur 

 on foliage. Other examples: 



Apple scab. In this disease the spots are produced on 

 the foliage and fruit making a distinctly scab-like appearance. 

 On the fruit the scab first appears as an olive green, circular spot. 

 This spot enlarges very slowly and causes damage because 

 it disfigures the fruit and exposes it to rot-producing fungi. 



Peach scab. This disease is characterized by small 

 circular spots on the young twigs, and by very small freckle- 

 like spots on the fruit which occasionally merge to form rough 

 blotches and possibly a cracking 'of the surface of the fruit. 



Practically all the scab diseases may be controlled by 

 spraying. 



Galls. Some plant diseases result in the formation of 

 large overgrowths of the host tissue. Club-root of the cab- 

 bage, crown gall of raspberries, fruit trees and other plants, 

 and the black wart of the potato are good examples. 



