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seasons and had been responsible, as explained below, for the 

 silvering of the foliage and the death of the branches, but 

 it was only after the latter had died that it proceeded to develop 

 externally and to produce its fructifications. 



Cause of the Disease. Although further research is 

 needed as to the nature of Silver Leaf, and many 

 problems connected with it await elucidation, authorities 

 are agreed that Stereum purpureum is capable of producing 

 Silver Leaf, and that this fungus is primarily responsible for 

 the disease in plum orchards all over the country. It is not 

 maintained that Silver Leaf is invariably brought about by 

 this fungus, but exceptions are comparatively rare and from the 

 fruit-grower's point of view they are negligible. 



The scientific knowledge which is possessed as to Silver 

 Leaf is due exclusively to the work of British botanists, and 

 for the last ten years Mr. F. T. Brooks of Cambridge has 

 made the disease a special study. This writer pointed out- 

 that the wood of a plum branch possessing silvered foliage 

 practically always shows dark brown or blackish markings 

 when cut across (see Fig. 4), although in the early stages of 

 attack this discoloration is usually considerably below the 

 silvered leaves. Microscopic examination revealed the fact 

 that the discoloured portion of the wood contained the fine 

 threads of fungus mycelium and that the brown colour was due 

 to the presence of a brown gum. Brooks further showed 

 that if portions of the mycelium of Stereum purpureum (derived 

 either from naturally-grown specimens or from the fungus 

 grown in pure culture) were inoculated into a healthy plum 

 under conditions which preclude the possibility of infection 

 from other sources, precisely similar mycelium was produced, 

 together with the discoloration of the wood and the formation 

 of gum. The foliage also developed the characteristic silvery 

 appearance. This experiment has been performed over and 

 over again on various kinds of plum and at different seasons 

 of the year with the same result. Inoculations with the spores 

 of the fungus also produced the same effect. No silvering of 

 the foliage, however, occurred if cuts were made but no 

 Stereum purpureum inserted, or if some other fungus (such 

 as Stereum hirsulum) was employed instead. It is important 

 to note that if the silvered foliage is examined no mycelium is 

 found in the tissues, nor as a rule is there any found in the upper 

 part of the affected branch and twigs, at all events in the 

 early stages of attack. The fungus is confined to the dis- 

 coloured portion of the wood which occurs lower down, though 

 its effects are seen in the silvering of the foliage. The silver- 

 ing itself is found to be primarily due to the accumulation of 

 air below the epidermal cells, much in the same way that, 

 owing to the presence of air, a white streak often shows in 

 a block of ice when it is cracked. The epidermal cells have a 

 tendency to break away from the cells immediately below, and 



