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 réle. 
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 aérial 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. 
