19^6.] "Canker" Fungus and Woolly Aphis. 55 



The fungus causing canker on apple trees, and less frequently 

 on pear trees, known as Nectria ditissinia, is a native of this 

 country, and also occurs on oak, beech, ash. 



Canker^' Fung-US ^^^el, alder, maple, and lime, hence any 



Woolly Aphis, attempt at extermination is hopeless. On 

 the other hand, it is within the power of all 

 those interested, by adopting proper precautions, to reduce the 

 destructiveness of this parasite in the orchard to a minimum. 



A series of careful observations, extending over several years, 

 has revealed the fact that, out of the trees enumerated above, the 

 apple is the only one on which the canker disease has assumed 

 the proportions ^of an epidemic. This increase of canker on 

 apple trees coincides with the rapid extension of the Woolly 

 aphis {Sc/iizojienra lanigerd) in this country. 



Canker fungus is a wound-parasite, that is, its spores on 

 germination cannot pierce the unbroken bark and enter the 

 living tissues of a branch, but can only effect an entrance 

 through a wound made by some other agent. The minute 

 punctures in the bark of young branches, also the gaping 

 wounds and large rugged excrescences on older branches made 

 by the Woolly aphis, are admirably adapted for enabling the 

 canker fungus to gain an entrance to the living tissues of the 

 branch. That this opportunity is not neglected is proved by 

 the fact that in nearly every instance where canker on an apple 

 tree is sufficiently recent to show how it gained an entrance, it 

 can be traced to a wound made by the Woolly aphis. 



The matter does not end here ; when a tree is once infected, 

 and the fungus produces spores, these are carried from one part 

 of the tree to another by the aphis, and eventually an epidemic 

 of canker is the result. 



Of course, wounds caused by other agents, such as hailstones, 

 the cracking of branches by wind, &c., furnish suitable starting- 

 points for the entrance of canker ; nevertheless, it is perhaps not 

 overstating the case to say that if we had no Woolly aphis we 

 should have no canker, at least not in the form of the destructive 

 epidemics so prevalent at the present day. 



Leaflets dealing with the Woolly Aphis (No. 34) and with 

 Canker (No. 56) can be obtained on application to The Secretary, 

 Board of Agriculture, 4, Whitehall Place, S.W. 



