12 



THE "CANKER" FUNGUS 



{Nedria ditissimd). 



The term " canker," as used by fruit growers, is applied to 

 any disease of fruit trees, independent of its origin, where the 

 bark becomes cracked and more or less destroyed. It is, 

 however, very important that the true cause of the canker 

 should be ascertained, otherwise preventive measures cannot 

 be applied with any certainty of success, as the remedy most 

 effective ag*ainst one particular form of canker may prove 

 useless against another kind of different origin. 



The most frequent and at the same time most destructive 

 form of canker attacking apple-trees in this country is caused 

 by a minute fungus [Nedria ditissima), and as the general 

 appearance of the wounds produced is very well marked no 

 difficulty should be experienced in recognising this particular 

 form . 



The fungus can only gain admission to the living portion 

 of a branch through a wound, being unable to pierce the 

 unbroken bark. In the case of slender branches the wounds 

 may be caused by frost, hail, or the punctures of insects. 

 Having once gained an entrance the fungus spreads rapidly 

 in the living bark, which becomes eaten away in irregularly 

 shaped patches, leaving the wood exposed. In some cases the 

 wound is confined to one side of the branch, but in many 

 instances the bark is completely destroyed all round the 

 branch, when the portion above the wound is at once killed. 

 In very young branches the wood is also frequently destroyed, 

 as shown in Fig. i. A very characteristic feature of the 

 disease, when attacking young branches, is the thick rugged 

 mass of bark which forms round the edge of the wound. 



On older parts of the tree canker _ usually first appears in 

 the fork of a branch, access being gained by the fungus 



