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 hyphe 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. 
