31 



not necessarily free the plant from the disease. As explained 

 above, the bacteria, in certain plants in any case, spread in the 

 tissues by means of the tumour strands and, if the gall is 

 merely cut off, the disease may break out again later. On 

 the other hand, if a deeper wound is made with the object of 

 cutting out the whole infected area, the wound may be more 

 serious than the gall itself and may injure or even kill the tree. 

 If the galls are on the lateral roots and only a few are attacked 

 the affected roots should be cut out, but if badly galled the 

 trees should be burned. 



SILVER LEAF IN FRUIT TREES 



(Stereum purpureum.) 



During recent years fruit growers have suffered very severe 

 losses from the disease known as Silver Leaf. The trouble is 

 particularly common on trees belonging to the Order Ros&cece, 

 for example, plum, apple, almond, apricot, cherry, peach, 

 nectarine, and Portugal laurel, but trees and shrubs belonging 

 to altogether different orders are also affected. Amongst these 

 may be mentioned currant, gooseberry, horse chestnut, labur- 

 num, and tree lupin. It is, however, to the plum that Silver 

 Leaf has caused most serious loss, the Victoria variety 

 especially having been attacked and killed in large numbers, so 

 much so that unless drastic measures are taken to prevent the 

 spread of the disease this valuable variety of plum is threatened 

 with extermination. It is also a frequent source of death to 

 flowering almonds in London suburban gardens. 



Description. As the name implies, the foliage of trees 

 suffering from Silver Leaf develops a silvery or leaden sheen 

 which usually commences in a single branch. After a time the 

 affected branch begins to die back, and as branch after branch 

 becomes attacked large portions of the tree die, with 

 the result that in time the whole tree is killed. The 

 length of time between the first silvering and the death of the 

 tree varies with the species or variety, the extent of the 

 attack, and other factors. If affected branches are removed 

 as soon as the first signs of attack are manifest, trees may 

 often be saved, and in a few cases affected trees recover 

 without any treatment whatever. As a general rule, however, 

 unless promptly attended to, the tree inevitably succumbs. 

 After the death of a branch, but not necessarily after the whole 

 tree has died, a purple fungus named Stereum purpureum 

 will usually be found pushing its way through the dead bark 

 and forming flat incrustations (see Fig. 2) or bracket-shaped 

 bodies (see Fig. 3) on the surface of the dead bough and 

 branch. These structures are the reproductive portions of the 

 fungus, and produce innumerable spores. The fungus itself 

 had been present in the wood of the tree for one or more 



