42 MANUAL OP TREE DISEASES 



and small-fruits, it has been studied in Europe since 1885 when 

 it was first described in France. Apple, plum, peach, apricot, 

 cherry, almond, currant, gooseberry and lilac are among the 

 trees and smaller woody plants known to be affected. The stone- 

 fruits are probably most seriously affected. The disease is 

 known at present in France, England, Germany, Canada, 

 northeastern United States, South Africa and New Zealand, 

 but probably has a much wider distribution. It has been 

 noticed jn the forest and on trees outside the forest in north- 

 eastern United States and the adjacent regions of Canada. 



Symptoms. 



The leaves of affected limbs are at first paler in color and 

 finally become milk-white, lead-colored or silvery. The green 

 color is not entirely lost but is only faintly evident in the silvered 

 areas. The disease shows in two distinct types depending on 

 the part of the tree first attacked. Usually the first symptoms 

 of silver-leaf are confined to a single twig, and then from year 

 to year other twigs, large branches and finally the entire tree 

 show the disease. Before the entire tree is diseased, however, 

 the twigs first affected die and the fruiting-bodies of the causal 

 fungus appear on them as small white and purplish encrusting 

 patches. At other times the roots are infected first and large 

 branches develop silvered leaves suddenly, and very soon the 

 entire tree becomes diseased. On cutting into the branches 

 which show silvered leaves, the wood will be found to be dark 

 brown. The connection between these symptoms is explained 

 in the discussion below. 



Cause. 



The cause of silver-leaf long remained a mystery, largely 

 because many scientists thought without experimentation that 

 it was due to simple physiological disturbances. However, 

 it has been definitely proved that the fungus, Stereum pur- 



