210 SUver-Leaf Disease, TTT 



6. EXPERIMENTS ON THE CURATIVE TREATMENT OF SILVER- 

 LEAF DISEASE. 



It was pointed out in 1913(2) that in the state of our knowledge 

 then, the only practicable means of control of the disease in fruit planta- 

 tions lay in the excisiqn of silvered branches and the eradication of 

 trees which were beginning to die back. These measures and the results 

 obtained by their application on a large scale will be referred to later 

 in this paper. The present section deals with experiments of a chemical 

 or quasi-chemical nature made in an attempt to seek a cure by medical 

 rather than by surgical means. 



Various workers have stated from time to time that they have cured 

 silver-leaf in fruit trees by certain forms of manurial or chemical treat- 

 ment, but upon enquiry it has been found that the number of trees so 

 treated has been small and sometimes one only. It is also now well 

 known that silvered trees not infrequently recover without treatment 

 of any kind, so that the value of experiments conducted with small 

 numbers of trees, especially where adequate controls are not kept, is 

 doubtful. The extent of natural recovery varies considerably from year 

 to year, but in our experience it was most marked in 1915 when a con- 

 siderable number of the plum trees used in these investigations recovered 

 naturally. 



For the most part, the plum trees experimentally treated as described 

 below, were in a moderately silvered condition as a result of inoculation 

 by Stereum purpureum, although none of them had begun to die back 

 at the time of treatment. 



(a) Treatment with artificial manures. 



In the Gardener's Chronicle for August 16th, 1913, "Southern 

 Grower" stated that the application of artificial manures had caused 

 the partial or complete recovery of several plum trees in one of his older 

 plantations. During the winter of 1913-14 the root systems of five 

 silvered trees in our plots were treated each with two pounds of a 

 mixture of two parts superphosphate, one part sulphate of ammonia, 

 and one part muriate of potash, the amount applied being less than 

 that used by "Southern Grower" as the trees were younger. In 1914 

 none, and in 1915 one, of these trees recovered, although in the latter 

 year several other untreated trees became healthy. 



Four other trees heavily dressed with farmyard manure, two re- 

 ceiving a second application, showed recovery only in one. 



