236 SCIENCE AND FRUIT GROWING 



disease is illustrated by the following values, though they do not 

 in all cases give a true measure of susceptibility, for the observa- 

 tions were taken two years after the inoculation, and, in the 

 case of the laurels, that had not allowed sufficient time for the 

 fungus to spread to its utmost. 



Plums . . -70 per cent, affected. 



Apples . . . 26 ,, ,, 



Laburnums . . . 17 ,, ,, 



Laurels . . . 25 ,, ,, ,, 



Pears . . . . II 



In these experiments stereum derived from the apple and 

 laburnum, as well as that derived from the plum, was used, and 

 it was found that the fungus caused infection in all these kinds 

 of trees whatever its origin might have been ; no definite evidence 

 was obtained to show that the fungus was in any way modified 

 by the nature of its host, or that it varied in potency according 

 to the source from which it had been derived. 



The susceptibility of different varieties of plum trees was 

 investigated in the case of the four varieties, Monarch, Czar, 

 Early Prolific and Victoria ; but no difference in their behaviour 

 was apparent : the percentages of trees, taking the varieties in 

 the order mentioned, which took the disease after inoculation 

 were 



78, 78, 69 and 70 ; 



and the severity of the attack was in the proportion of 

 36, 57, 70 and 63. 



These results gave no support to the view, held by certain 

 authorities, that soft -wooded varieties are the most susceptible ; 

 yet it is very probable that soft -wooded varieties, under the 

 conditions existing in a plantation, suffer most, for they are more 

 liable to mechanical injury, and hence present more frequently 

 wounds through which the fungus spores may obtain an entry. 

 Even the injury caused by aphis may be sufficient to afford a 

 means of access for the spores : and this, also, is probably the 

 origin of another opinion, which is sometimes expressed, that 

 aphis causes silver-leaf. 



It is probably due to the liability to mechanical injury which 

 would follow on rampant growth produced by over-feeding, that 

 such over-feeding has been assigned as a cause of silver-leaf ; for 

 vigorous growth in itself does not appear to favour the disease : 

 in a separate series of experiments with the same four varieties 



