ii8 PLANT DISEASES 



WHITE ROOT-ROT 



(Rosellinia necatrix. Prill, and Del. 

 = Dematophora necatrix, Hartig.) 



Fortunately this scourge is rare in Britain, although too 

 well known on the Continent, where it attacks vineyards, 

 orchards, etc., in a wholesale manner. One of the marked 

 peculiarities of the present fungus is its power of becoming 

 parasitic upon a large number of plants belonging to widely 

 separated Orders ; in fact, it may be stated broadly that it 

 attacks every plant with which it comes in contact. 



Hartig enumerates the following as having been destroyed 

 by the fungus : vines, fruit-trees, potatoes, beans, beet, 

 young maples, oaks, beeches, pines, and spruces. As with 

 Armillaria mellea, the mycelium of the fungus under 

 consideration spreads rapidly underground, and when it 

 comes in contact with the rootlets of a plant it kills them, 

 and gradually works its way into the tissues of the larger 

 branches of the root. In the case of large plants, the 

 mycelium, after travelling along the tissues of the root up 

 to the base of the trunk, bursts through the cortex in the 

 form of a snow-white, fluffy mycelium, which again enters 

 the ground and spreads in search of fresh victims. 



During the progress of the disease numerous minute 

 sclerotia are formed in the cortex of the diseased roots ; 

 and if such roots happen to be exposed to the air, these 

 sclerotia come to the surface and give origin to groups 

 of minute, bristle-like, dark-coloured conidiophores which 

 bear numerous conidia at their tufted tips. 



A second kind of fructification sometimes occurs on 

 decaying roots, under the form of minute, black con- 



