PLUM DISEASES 371 



nation. After affected trees have died, fruiting bodies of the 

 causal pathogene appear (Fig. 108). The limbs and roots are 

 affected, but give no external evidences of the disease. . But on 

 cutting into these parts the wood will be found to exhibit a 

 brownish discoloration. Frequently discolored hearts are not 

 accompanied by silvery foliage, and vice versa. In certain 

 cases the discoloration of the wood is due to some other cause 

 than the silver-leaf pathogene. As a general rule only about 

 33 per cent of the affected trees recover from the effects of the 

 disease, although cases are on record where a tree once entirely 

 silvered finally recovered. 



Cause. 



Although the silver-leaf disease has been known for many 

 years, the true cause was not definitely determined until 1902, 

 when the fungus Stereum purpureum was discovered in this role. 

 Some had held that bacteria were the causal factor involved, 

 while others dismissed the question with the statement that the 

 silvering of plum leaves was the result of some disturbance in 

 the nutrition. Other more or less fantastic speculations regard- 

 ing the cause of the disease are encountered in a review of the 

 notions held on the subject. 



Trees become inoculated above and below the ground ; that 

 is, the fungus may enter the tree through the branches or trunks, 

 or through the roots. In general the trunks are the more liable 

 to attack. Spores are blown to the aerial woody parts, where 

 they germinate and penetrate through a wound of some sort. 

 The fungus is not carried by pruning tools ; it is not dissemi- 

 nated by means of buds or scions from a diseased tree. How- 

 ever, it should be noted in connection with the latter point that 

 scions from healthy trees when grafted on diseased stock become 

 diseased, as evidenced by their silvered leaves. The spores, 

 which are carried to the trunks and branches, may come from 

 any of the long list of plants which happen to be dead as a result 

 of the attacks of Stereum purpureum. The fruit-bodies (Fig. 



