182 



STUDIES ON THE APPLE CANKER FUNGUS. 

 I. LEAF SCAR INFECTION^ 



By S. p. WILTSHIRE, B.A., B.Sc. 



{University of Bristol Agricultural and Horticultural 

 Research Station, Long Ashton, Bristol.) 



(With 2 Text-figures and Plate III.) 



For some years an investigation of the biology of the apple tree canker 

 fungus has been proceeding at Long Ashton (i), and the work has now 

 reached a point when the results can be published in detail. This paper 

 will be confined to the infection of the leaf scars by the fungus. The 

 ultiniate object of the investigation was to find some means of control 

 of this disease, and as a preliminary step it was necessary to find out how 

 the fungal parasite entered the living tree. 



Only incidental references are made to the hterature of the subject 

 as a summary of the work already published is given by Cayley(2). 



The apple canker fungus is usually spoken of as a "wound parasite," 

 and it is commonly supposed that, provided no agency is allowed to 

 wound the host, infection is impossible. In the case of the canker fungus 

 this view needs modification, since this fungus enters the normal healthy 

 tree without any previous external injury. In fact, a most important 

 path of infection is through the scars left by the fallen leaves. Nectria 

 ditissima^ therefore is only a wound parasite, if we include in the term 

 "wound" such natural ruptures of external tissues as those occurring 

 in the normal growth of the plant. 



INCIDENflE OF LEAF SCAR INFECTION. 



The infection of canker through leaf scars is responsible for a large 

 percentage of the canker at Long Ashton. This type of infection occurs 

 on many varieties and is regular in its incidence, whilst from material 

 received from South Devon, Worcester and Somerset, it would also 

 appear to be widely distributed. In cider orchards (which under normal 



1 A grant in aid of publication has been received for this communication. 

 - No opinion is expressed regarding the name of Nectria galligena, Bres. recently 

 suggested for tlie canker fungus. 



