APPLE-TREE CANKER 127 



species of Poa^ Holcus, F/ileum^ etc. If useless grass grow- 

 ing in damp places is cut as soon as the disease manifests 

 itself, its spread is much retarded, 



Tulasne, Fungi Se/. Carp., vol. iii. p. 24. 



Prillieux, Malad. des Phmtes Agric, vol. ii. p. 96, figs. 



APPLE-TREE CANKER 



{Nectria ditissima, Tul.) 



This parasite by no means confines its attacks to the 

 apple, but is equally common on the beech, and, according 

 to Hartig, also invades the oak, hazel, ash, hornbeam, 

 alder, maple, lime, dogwood, and bird-cherry. 



The fungus is usually a wound-parasite, its conidia or 

 spores germinating in the first instance on some wounded 

 surface, and from thence invading the healthy tissues. 

 The bark is first attacked and destroyed, often cracking in 

 a concentric manner ; afterwards the wood is also destroyed, 

 the canker often completely girdling small branches. As 

 a rule a thickened, irregular margin of living bark forms 

 round the wound, giving a rugged appearance to the parts 

 attacked, and very characteristic of the disease. Not un- 

 frequently several diseased spots appear on a branch at 

 places where there is no evidence of external inoculation, 

 and Hartig explains this by assuming that when the fungus 

 has once gained admission to the living tissues through a 

 wound, the mycelium travels in the substance of the 

 branch, bursting through the bark at intervals to form its 

 fruit on the surface. The fungus is somewhat capricious, 

 not attacking the margin of the wound equally all round, 

 but often concentrating its work of destruction at one 



