208 Silver-Leaf Disease, III 



(b) One of the main roots was severed and the cut end was immedi- 

 ately connected by tubing to a reservoir placed as above. This was 

 found to be the most satisfactory method. 



Apart from one tree, silvering wa.s not induced by injecting an ex- 

 tract in this way, and the results being almost entirely negative, one is 

 dubious about claiming that this single tree became silvered on account 

 of injection. 



Cut shoots of healthy Victoria plums placed in a similar extract 

 during March and kept as far as possible under sterile conditions, de- 

 veloped their buds normally and the leaves did not become silvered. 



Various other substances have been injected into Victoria plum trees 

 in the hope that thereby silvering might be induced, but without success. 



Efforts have also been made to induce silvering by checking or 

 otherwise altering the transpiration current. Thus young Victoria plum 

 trees have been drastically root-jiruned, branches have been snapped 

 nearly asunder during spring, and the stems of young trees have been 

 cut back so as to leave only a few buds to develop, without silver-leaf 

 developing. 



5. FURTHER OBSERVATIONS ON THE MANNER OF INFECTION IN 

 FRUIT PLANTATIONS. 



Our views in this respect have undergone no material change since 

 the publication of the second paper of this series in 1913, in which by 

 far the most important cause of silver-leaf disease in the fruit planta- 

 tions of this country was considered to be the fungus, Stereum pur- 

 pureum acting as a parasite which entered through wounds in the stems 

 and branches of the trees. 



The possibility of root infection was not excluded where diseased 

 roots came in contact with healthy ones or where roots became exposed 

 and wounded from some cause or other, but we have not yet observed 

 any trees, which indubitably became infected in this way. Wounds 

 made during grafting or budding are an obvious source of danger. 

 Plum trees in an advanced state of the disease often throw up suckers 

 which are silvered on account of the passage of the fungus into the 

 subterranean parts and Hector (6) has stated that in some parts of 

 Middlesex such silvered suckers have been used as stocks upon which 

 Victoria and other varieties were "worked." It is clear that if a silvered 

 sucker used as a stock contained discoloured wood bearing the mycelium 

 of Stereum purpii renin, there would be a probability that the scion would 

 also become attacked. It by no means follows, however, that silvered 



