34 



Where the roots of two trees overlap the mycelium from a 

 diseased root may attack a sound root if in actual contact 

 with it, and thus spread the disease. The mycelium, however, 

 does not spread through the soil itself, but in or along portions 

 of the woody roots. According to Brooks, however, it is not 

 by means of the roots that the disease is generally spread, and 

 in support of this it will be noted that affected trees, for in- 

 stance in a plantation of Victorias, usually appear scattered 

 about in an irregular manner, and not radiating out from 

 definite common centres. 



Silver Leaf is especially prevalent in trees which have been 

 cut down and regrafted, and in the case of apples it is as a 

 rule only top-grafted trees which suffer to any large extent. 

 The varieties badly attacked when top-grafted are mostly those 

 which are known to callus poorly, such as Lord Sumeid, Lord 

 Grosvenor, Manx Codlin, Ecklinville and Pott's Seedling. The 

 explanation, doubtless, is that the unprotected cut stump offers 

 exceptional facilities for spore-infection. This infection may 

 not take place immediately but if it does occur and mycelium 

 develops in the stock it usually spreads to the graft and brings 

 about its rapid death. Fig 5 shows a case where the fungus 

 obtained an entry soon after grafting and where the scions 

 made little growth. 



In connection with spore-infection it should be remembered 

 that the spores which give rise to new infections do not neces- 

 sarily originate from a tree of the same kind. Stereum 

 purpureum, for instance, is often found on laburnum and 

 poplars in gardens, and Brooks has shown that spores from 

 these host-plants may bring about Silver Leaf in plums. 



Susceptibility of Varieties. All varieties of plum are 

 liable to Silver Leaf. At one end of the list stands 'Victorian, 

 by far the most susceptible of all, with Czar (very subject 

 to the disease in certain localities) as second, and at the other 

 end River's Early Prolific and the Yellow Pershore, which are 

 highly resistant. The explanation of the frequency of attack 

 in Victoria has been sought in its heavy cropping powers which 

 not only weaken the trees, but bring about breaking of the 

 branches, thus affording points of entry for the fungus. This 

 is, however, probably only partly responsible, the true explana- 

 tion doubtless lying in those subtle factors which govern sus- 

 ceptibility and immunity in all kinds of plants and animals. 



The question of the effect of stocks has recently attracted 

 attention and in some quarters it has been stated that| 

 Victorias grafted on highly resistant varieties partake of the 

 resistance of the stock. Many trees have consequently been 

 worked recently on the Yellow Pershore Plum, especially in 

 the Evesham district, but whether a permanent beneficial effect 

 has been produced or not is at present uncertain, since the use 



