F. T. Brooks and M. A. Bailey 209 



suckers arising from diseased trees necessarily contain discoloured 

 wood themselves and, if they do not, there is no reason why these 

 suckers used as stocks should not grow out of the malady. 



The practice of using silvered suckers as stocks cannot be condemned 

 too strongly, but it is not one which reliable nurserymen would follow 

 under any circumstances. In our own experience we have only rarely 

 seen plum trees under five years of age silvered and the danger of the 

 disease being spread from reputable nurseries in the manner described 

 above is probably negligible. 



It is not usually until a plum orchard is in full bearing that silver- 

 leaf plays havoc on an extensive scale even with the most susceptible 

 varieties. From the age of twenty years onwards is the most critical 

 time, and, as has been pointed out in the earlier papers, single branches 

 of trees are usually first attacked, the earliest trees to become affected 

 being scattered in the plantation. 



If the first trees to be attacked are neglected and allowed to die 

 back with the result that fructifications of Stereum fwjiurevm develop, 

 other trees adjacent to these may fall a prey with the result that groups 

 become affected and simulate an attack by a root parasite which spreads 

 through the soil. 



As is well known, the variety Victoria is the most susceptible to 

 silver-leaf disease of all plums grown in this country, and perhaps the 

 variety Pershore is the most resistant. It has been thought that the 

 former variety was particularly susceptible perhaps on account of the 

 frequent breaking of the branches through overloading with fruit, there- 

 by providing wounds for the fungus to enter. This idea is, however, 

 invalidated by the remarkably brittle character of the old branches of 

 the Pershore plum, trees of which are often built up, as it were, anew, 

 by the development of young branches from the lower parts of the tree. 

 In mature plantations of Victoria and Pershore plum there would 

 probably be more wounds available for Stereum fitrfurenm in the latter 

 than in the former. The explanation of the special susceptibility of the 

 variety Victoria is probably more subtle and is more likely to be found 

 in some difference in the character of the wood itself. In horticultural 

 parlance, the Victoria is known as a soft-wooded, and the Pershore as a 

 hard-wooded variety, and this difference in texture, which signifies 

 much, is probably concerned in this matter. 



