THE BOTANY OF THE APPLE TREE. 



31 



Black Heart. Both branches and twigs are subject to this disease, 

 which is discussed under the "Diseases of the Trunk.' 7 



DISEASES OF THE LEAVES. 



Powdery Mildew. The leaves of young trees, especially those in the 

 nursery, are quite subject to the attacks of a powdery mildew which 

 covers their surface with a white powdery coat. The leaves soon be- 

 come somewhat curled and wrinkled, and eventually are greatly in- 

 jured. The fungus which causes this disease is one of the Simple 

 Sac-Fungi (Perisporiacece) and bears the scientific name of Podos- 

 phcera oxycanthce. It consists of slender, white, branching threads 

 which creep over the skin of the leaf, sending in little suckers here and 

 there, which take food matter from the cells. At length many 

 branches are sent up at right angles to the surface (Fig. 6, A). These 



wr 



FIG. 6. The Powdery Mildew. A, fragment of a thread 

 with^a vertical branch producing summer spores; B, 

 spore-sac from the fruit; C, one of the ripe fruits with 

 its radiating appendages. All much enlarged. 



quickly break up into summer spores, and as these form in great num- 

 bers they give the leaves the well-known white-powdery appearance. 

 The summer spores blow away in the wind and under favorable con- 

 ditions germinate quickly and give rise to a new growth of the fungus. 

 Later in the season, usually not until the autumn, the fungus produces 

 its small spherical fruits (Fig. 6, C), which are black in color and 

 barely large enough to be seen with the naked eye. 



