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LIX.—BLACK-KNOT OF BIRCH 



G, Massee* 



A disease attacking bircli trees is just now much in evidence in 

 some parts of Scotland, and is probably widely distributed in this 

 country, as specimens collected at Eltham in Kent, and in York- 

 shire are now in the Kew herbarium. The disease is also present 

 in Sweden, Switzerland, Finland and Germany. A fungus 

 called Plowrightia virgultorumy Sacc, is the cause of the disease. 

 The genus PloivTightia includes some destructive parasites, 

 amongst others P. ribesia, Sacc, producing Gooseberry black- 

 knot, not uncommon in this country, and P. Tnorbosa^ Sacc, the 

 cause of the dreaded black-knot disease of plum and cherry trees 

 in the United States and Canada. The last named disease is at 

 present unknown in Europe. Shoots and branches of birch from 

 one to four years of age are attacked, the fungus gaining an 

 entrance through the lenticels. After infection a black stroma 

 is gradually formed which extends completely through the thick- 

 ness of the cortex, but is checked when it reaches the wood. The 

 diseased patches are narrow, and elongated in the direction of 

 the long axis of the branch, varying from a quarter of an inch 

 to an inch in length by two lines to a quarter of an inch in breadth. 

 During the first season of infection the surface of the stroma is 

 flush with that of the shoot, and is covered with a dense blackish 

 olive layer bearing myriads of very minute conidia. At a later 

 stage the stroma increases in thickness, .becomes hard and 

 carbonaceous, and produces the higher or ascigerous form of fruit. 

 As the branch continues to increase In thickness after infection, 

 the patches of fungus become surrounded by a wall of tissue, and 

 eventually present the appearance of blackened patches occupying 

 the bottom of a more or less deep, elongated pit. Owing to the 

 facility for infection, due to the lashing together by wind, of 

 healthy and diseased branches, the points of infection are usually 

 numerous on each shoot, and are readily distinguished by the 

 gouty or knotted appearance presented. The first indication of 

 disease is the wilting and yellowing of the leaves, followed by the 

 death of the branch. Trees that have been diseased for some time 



present a peculiar and characteristic appearance. Owing to the 

 death of the terminal shoot, numerous laieral branches are pro- 

 duced, the lead of each one in turn being killed by the fungus ; 

 a repetition of this process results in the production of more or less 

 dense tufts of stunted branches bearing a superficial resemblance 

 to "witches' brooms. '* 



