33 



the presence of air interferes with the normal reflection of 

 light from the surface of the leaf. 



From corespondence in the horticultural press it is evident 

 that many persons have not appreciated the above facts and 

 are still concerned with theories and speculations as to the 

 cause of Silver Leaf, or occupied with side-issues. There is 

 occasionally a silvering of plants, due to an unknown cause, 

 which simulates Silver Leaf, and the loss of colour in leaves 

 suffering from a bad attack of red spider or aphis may also 

 sometimes be confused with the true disease. It should be 

 clearly understood, however, that Stereum purpureum is the 

 responsible parasite in the fruit plantations of the country, and 

 that no other theory which has been put forward to account 

 for Silver Leaf in association with the dying back of fruit 

 trees has been proved. 



Description of the Fungus. It is important that growers 

 should learn to recognise the fungus causing the disease, as it 

 may occur on the dead wood of other trees besides plum, and 

 should not be allowed to persist in gardens. By the Silver 

 Leaf Order, moreover, all wood harbouring it must be burned. 

 (See last paragraph of this leaflet.) 



The fructifications of Stereum purpureum are purple-mauve 

 when fresh, often with a white or pale woolly margin, but 

 they change colour with age. In consistency they are leathery. 

 They are very variable in form and appear either as flat in- 

 crustations up to several inches long covering the under surface 

 of the branches or on the sides of the trunk (see Fig. 2), or as 

 bracket- shaped projections of -in. to 1-in. in width, and 

 arranged in tiers one above the other (see Fig. 3). In this case 

 the upper surface is hairy, and the under surface smooth. The 

 purple colour, however, is the characteristic feature and no 

 other fungus of this colour occurs on plum. 



The spores are produced in abundance on the smooth under 

 surface of the fructifications. Although the latter shrivel up 

 in dry weather they are capable of reviving with rain and dis- 

 .charging a fresh crop of spores. In this way the spore-discharge 

 from a given fruit b'ody may last over a long period. The 

 fructifications appear at any time of year when the weather is 

 moist and mild, but they are produced in the greatest abun- 

 dance after the heavy rains of autumn. 



Method of Spreading The fungus is propagated by the 

 spores whicE are freely distributed by wind. They germinate 

 readily in moist weather, and the fungus gains admission to 

 the trees through wounded surfaces such as cracked branches, 

 injured trunks, fissures in the bark, and any other unprotected 

 wounds. On germination, a mycelium is produced which first 

 of all attacks the dead and injured tissues, but subsequently 

 invades the water-conducting cells and attacks the living part 

 of the tree. It has been shown that infection by spores cannot 

 take place through the sound and uninjured bark. 



