232 SCIENCE AND FRUIT GROWING 



only necessary to insert a small portion of this fungus, rendered 

 active by having been kept in a warm, moist atmosphere, in a 

 cut made in the bark of the tree, and binding up the cut so as 

 to prevent the fungus from being washed out. 



The first sign of infection consists of a gradual appearance 

 of silvering of the leaves, which, when the tree is in active growth, 

 may occur in two or three weeks after the inoculation, whilst 

 the whole tree may become seriously affected within four or 

 five weeks. In many cashes it is noticeable that the infection 

 seems to spread from the point of inoculation : if one branch 

 only has been inoculated, that branch is the first to surfer, and 

 if one side of the stem has been inoculated, it is on that side that 

 the silvering first appears. 



The outward manifestation of disease is, no doubt, due to a 

 poison formed during the growth of the fungal threads probably 

 this is so with all fungoid and bacterial diseases for no evidence 

 of the presence of the fungus itself, i. e. its mycelium, can be 

 found in the leaves. The silvery appearance is but one of the 

 manifestations of the disease, and seems to be due to the cells 

 composing them having become partially disconnected, owing 

 to changes brought about in the nutrition of the plant. There 

 is no reason for supposing that a similar silvering might not be 

 produced by other means, and there is some slight evidence that 

 this is occasionally so, 1 but no argument could be based on such 

 a fact (if established) in support of the view that the disease 

 known as silver-leaf disease is not of fungoid origin. The change 

 noticed in the leaves is often accompanied, though not always, 

 by a change in the wood, this becoming dark and discoloured, 

 even to the centre of the branch, and the discoloration in partial 

 attacks extends below the lowest point at which the leaves show 

 silvering. 



Silver-leaf disease is conspicuous in its attacks on plum trees 

 and other stone fruits, but it appears capable of attacking almost 

 any tree or shrub. It has been noticed on apples, pears, goose- 

 berries, currants, laburnums, Portugal laurels, walnuts, birch, 

 beech, horse chestnut, sycamore, spiraea, white dead nettle, etc. 2 



The spread of the disease amongst plum trees in this country 

 became serious in about 1904, due, probably, to the exceptionally 

 wet character of the summer of 1903. In New Zealand, where 

 the disease has caused much damage, its attacks appear to have 

 become acute in about 1900. Whether in either of these 



1 Brooks, Journ. Agric. Science, V, 293, 304. 



2 Cf. Brooks, loc. cit., IV, 133; V, 288. 



