372 MANUAL OF FRUIT DISEASES 



108) develop only on dead trees, either standing or in the 

 brush-heap. They also occur on stumps of trees once silvered. 

 So far as root-infection is concerned, the mycelial strands are 

 easily carried in cultivation; they are torn from the roots of 

 affected trees, transferred to roots of neighboring trees, where 

 entrance is gained through an injury made by the cultivator. 

 As intimated above, leaves are never directly affected. Trees 

 infected in the fall show signs of the disease in the spring of the 

 following year. Those infected earlier in the spring or summer 

 may show silvering within one or two weeks, or in some cases 

 only after two months. In all cases of branch-infection the 

 leaves above the infection-court show more silvering than those 

 below the point of attack. And only those leaves on the same 

 side as the infection-court show the disease, those off a straight 

 line from the point of inoculation to the tip remaining normal. 

 It appears then that the real disturbing factor concerned in the 

 silvering of the foliage is conducted rapidly in the sap. This is 

 further supported by the fact that the mycelium of the fungus 

 has never been observed within the leaf-tissues. The hyphse are 

 found, however, in the roots, trunks and branches, in the xylem 

 vessels. Here the walls of the vessels and of the medullary 

 ray-cells are brown and their lumina are filled with a brownish 

 red substance. The cause of the silvering in the leaves is 

 thought to be due to some poisonous substance secreted by the 

 mycelium in the woody portions. There is no alteration of the 

 chloroplastids in the leaf -cells ; the normal structure of the leaf 

 is not markedly changed. The epidermis is enlarged, and raised, 

 the elevation resulting in the formation of air spaces between the 

 epidermal and mesophyl layers. Thus the chlorophyl cannot 

 show through the surface; instead, the air spaces give to the 

 leaf surface a lead-colored or silvery appearance. 



As previously noted, most trees die, after which the fungus 

 develops its fruit-bodies. These are long-lived, being able to 

 withstand at least 13 months of dry weather. On moistening 

 again, mature spores are ejected. 



