244 



* 



soaking for 4-6 hours is sufficient. The excess of picric acid is* 

 washed off and the section stained with concentrated Delafield 

 haematoxylin (ripe solution) for at least 4-6 hours, or preferably 

 overnight, according to the nature of the material. Picric acid 

 stains lignified tissues a yellow-citron, and haematoxylin imparts 



a blue colour to the hyphae. 



In the following account a description is given of the diseased 

 condition as found in the following plants, in which the hyphae 



pur pur eu 



II. — Specific Cases. 



er plum and other trees in the neighbour- 

 met with in the orchards of this locality 



(1) Victoria Plum. — The Plum orchards in the Brentford and 

 Heston district of Middlesex suffered severely from Silver leaf 

 during 1918; eighty to ninety per cent, of the plum trees (of ages 

 from 5-30 years), chiefly the varieties " Victoria M and " Czar," 

 were so severely affected that they had to be cut down during 

 late summer and early autumn. Owing to a scarcity of labour 

 the sawn-off branches and tree-stumps were not removed from the 

 plantations. The first fructifications of S. purpureum developed 

 during the last week of September on this dead wood and also on 

 silvered trees still standing. Undoubtedly diseased wood in the 

 plantations will endang 



hood. The plum trees met with in the orchards of this locality 

 were mainly grafted on the " Brompton " and " common plum " 

 stocks. Generally where suckers grew up from the stocks of 

 silvered plants, they showed silvering relatively to the same 

 extent as the scion, and this irrespective of the age of the plum 

 tree. In a few cases, however, I noticed normal suckers of 

 "Brompton" and "common plum" stock coming up from 

 silvered trees. Healthy suckers were most common on partially 

 silvered plants, that is, where onlv localised branches of a tree 

 were attacked, mainly where only the upper set of branches 

 was affected by the disease. I also noticed on a few completely 

 silvered trees healthy suckers of the above varieties arising from 

 the stocks. Silvered suckers, one year old, to the number of 

 150, were dug up and their roots examined; 10 of these were 

 taken to the laboratory, and 140* were examined in the field. 

 Of the latter 26 appeared normal to the naked eye in roots and 

 stem, whereas the remaining 116 revealed discoloration of the 

 tissues in cross section of roots and at the base of the stem. A 

 few two-year-old suckers were also examined and the discolora- 

 tion was still more obvious. The discoloration in the 10 suckers 



mentioned above could easily be traced up the suckers for two- 

 thirds to three-quarters of their length. The discoloration 

 usually consisted of 2-4 reddish brown streaks which sometimes 

 ran parallel, but frequently followed a winding or twisting path 

 in the stem. The streaks were confined to the outermost xyleni. 



The following table indicates the height reached by the dis- 

 coloration and by the hyphae in five one-year-old suckers. 



* These 140 suckers have been planted in the experimental plot of the 

 Royal Horticultural Society Gardens, Wisley. 



