DISEASES OP PLANTS. 349 



It is recommended that in tliis treatment lioles be bored in the trunk or 

 limbs to a depth of about half the diameter of the trunk or limb, and inclined 

 at an angle of 45°. This will facilitate the introduction of the sulphate of iron, 

 after which the holes should be closed and covered with grafting wax. So far 

 as the author's investigations have gone, they indicate that the operation should' 

 be carried on at about the end of July. 



Silver-leaf disease, Duke of Bedford and S. U. Pickering {Woiurn Expi. 

 Fruit Farm Rpt., 12 (1910), pp. 1-34; 'rcv- in Qard. Chron., 3. ser., 1,8 (1910), 

 No. 1246, pp. 356, 357, figs. 7). — A description is given of inoculation experi- 

 ments with this disease, which attacks many kinds of trees, especially plum and 

 other stone fruits. 



The disease is of long standing in England, and manifests itself by the foliage 

 of infected trees becoming light and silvery in appearance. Later the trees 

 may die, although death may not occur for several years, during which time 

 the diseased trees may still bear some fruit and show considerable vigor of 

 growth. 



In one series of experiments 48 2-year-old plum trees were inoculated by 

 inserting under the bark pieces from 2 to 3 mm. in diameter of the sporophore 

 of Stereum purpureuni obtained from a plum tree killed by silver leaf. Of 

 these 39, or 81 per cent, showed silvering before the end of July, 4 of which 

 were killed outright, 2 partially killed, 8 others entirely silvered, and the 

 remaining 2.5 only partially silvered. Of 64 uninoculated trees, not one showed 

 any signs of silvering. Nineteen of the inoculated trees died during the first 

 2 years after planting, all of which developed jS. purpurcum on the dead wood, 

 while none of the uninoculated trees died or showed any signs of the fungus 

 during this period. 



The results are also given of inoculating different parts of the trees, such as 

 stems, branches, and roots, and of inoculating various kinds of trees, such as 

 plums, apples, laburnums, laurels, and pears, with Stereum from various 

 sources. None of the trees tested were found immune to the disease, although 

 no Stereum could be obtained from the silvered laurels. 



No trace of the fungus hyphje was found in the silvered leaves, nor could 

 Infection be produced by using crushed silvered leaves as inoculating material; 

 neither could trees be infected by alternately using pruning tools on the living 

 parts of diseased and healthy trees. Attempts to graft diseased scions on 

 healthy stocks and healthy scions on diseased stocks were unsuccessful, as 

 no juncture between scion and stock would result. 



Apparently the disease can not be communicated from one tree to another 

 through the roots or from the soil, as diseased and healthy trees were grown 

 in such close proximity that the roots must have intermingled, while healthy 

 trees planted in the position formerly occupied by diseased trees showed no 

 special tendency to the disease. 



The silvering of the leaves on the inoculated trees may occur in one week 

 from the date of inoculation, while the entire tree may become seriously 

 affected within 4 or 5 weeks. 



Soft wooded varieties of plums seemed to be more susceptible to this disease 

 than any other, although in a plantation of many thousands of trees only 2.S 

 per cent were affected, while other varieties showed from 0.1 to 0.4 per cent 

 of silver leaf. 



As a result of these experiments the authors claim that S. purpureum is 

 undoubtedly the primary cause of silver leaf, and that the outward manifesta- 

 tion of the disease is due to a poison formed during the growth of the fungus, 

 while the silvery appearance of the leaves is caused by their cells becoming 

 partially disconnected, owing to changes bi'ought about in the nutritio)i of the 



