LEAF DISEASES AND INJURIES 43 



"pureum, which grows in the wood of the tree is the cause of the 

 disease. Inoculations from pure cultures of this fungus made 

 into the wood of roots, trunks and branches cause the develop- 

 ment of typical silver-leaf. The fungus occurs as a common 

 saprophyte everywhere and produces its fruiting-bodies on 

 dead wood, stumps and brush of all kinds. Spores from the 

 fruiting-bodies may cause infection of the exposed wood of 

 healthy trees. Less often the roots of trees may be invaded by 

 the mycelium. The mycelium grows in the heartwood, causing 

 it to turn uniformly brown. When infection occurs in the wood 

 of branches, the leaves just above the point of infection show 

 silvering the next season and in three to six years the remainder 

 of the foliage of the tree may become affected. When examined 

 microscopically, the silvery appearance of the leaves is found to 

 be due to the enlargement of the epidermal cells. This results 

 in the loosening of the epidermis and the formation of an air- 

 space between it and the palisade cells which contain the green 

 coloring matter. The other cells of the leaf are only loosely 

 held together and it is presumed that a ferment, produced by 

 the fungous hyphse growing in the wood, is transported to the 

 leaves and dissolves the cementing layer between the cells. The 

 mycelium itself does not enter the leaves but is confined to the 

 dark colored wood. Silvering never occurs without the pre- 

 vious browning of the wood. When the branch dies, Stereum 

 purpureum and other saprophytic fungi may soon develop their 

 fruiting-bodies on it. 



Control of silver-leaf. 



Since the fungus causing silver-leaf is a wound parasite, some 

 degree of protection may be afforded healthy trees by protecting 

 wounds with dressings (see page 348). Diseased limbs should be 

 cut from affected trees to the extent that no parts showing the 

 brown discolored wood are left ; otherwise the fungus will pro- 

 ceed in its growth into the trunk and out into other branches. 



