LEAF DISEASES AND INJURIES 41 



For further details and references in literature concerning the 

 leaf-cast diseases, see the discussions under pine diseases (pages 

 270 and 271) and fir diseases (page 159). 



Sooty Molds 



Caused by fungi of the family Perisporiaceaa 



The leaves of many trees are often covered with a 

 black sooty coating in midsummer. No serious damage is 

 done to the tree unless the growth on the leaves becomes so 

 abundant that it interferes with the functions of the leaf by 

 shutting out the sunlight. The fungi causing this type of growth 

 on leaves are closely related to the powdery mildew fungi (see 

 page 34) . The mycelium of the sooty mold fungi is, however, 

 dark colored and appears to the eye as black. As in the case of 

 the powdery mildews, the mycelium is superficial. No feeding 

 rootlets are sent into the leaf-tissue, however, as in the powdery 

 mildews, and the fungi are therefore not parasitic. They 

 obtain sufficient nourishment from sugary solutions that are 

 exuded from the leaves. Sugary exudations are only formed 

 on leaves under certain conditions, and when these are absent 

 the sooty molds do not occur. When aphids are present, sooty 

 molds find excellent conditions for developing. These fungi 

 may be controlled by spraying, and the recommendations given 

 for powdery mildews should be followed (see page 37). 



Silver-Leap 



Caused by Stereum purpureum Fries 



Although the disease known as silver-leaf or silver-blight 

 has not been shown to be as widespread or destructive on f orest- 

 and shade-trees as it is on fruit-trees, nevertheless it is known to 

 occur on ash, chestnut and others. As a disease of fruit-trees 



