275 



STUDIES ON THE APPLE CANKER FUNGUS 



II. CANKER INFECTION OF APPLE TREES 

 THROUGH SCAB WOUNDS i 



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



( University of Bristol Agricultural and Horticultural 

 Research Station, Long Ashton.) 



(With Plate XII.) 



Introduction. 



In a previous paper (i) reference was made to the fact that the canker 

 fungus Nectria galligena, Bres., can enter the apple tree through the 

 wounds caused by the scab fungus Venturia inaequalis. It is the purpose 

 of tliis paper to describe this process in detail. 



Symptoms. 



The scab fungus infects the shoots of susceptible varieties of apples 

 during the autumn and winter following their growth, the first infections 

 usually being found before the trees defoliate. In the spring most of the 

 pustules are surrounded by a cork layer and are subsequently completely 

 excluded from the tree, the only trace of the infection finally being a 

 shght roughness of the bark. 



Sometimes, however, this course of events is disturbed. The cortex 

 round the small scab pustule shows signs of blackening, and this is 

 accompanied in some cases by a swelling of the bark due to the growth 

 of the tissues beneath the infection (see PI. XII, fig. 1). Very early stages, 

 in which the discoloration is extremely slight, can sometimes be identified. 

 When the canker fungus has once got in (for as will be seen later this 

 difference from the normal development is due to Nectria galligena, 

 Bres.) it usually develops so rapidly that an area about 5 mm. in 

 diameter is completely killed and blackened before any attempt at 

 phellogen formation becomes effective. The canker area is usually 

 somewhat sunken, there is no crack in the bark between the healthy 



^ A grant in aid of publication has been made for this communication. 



18- -2 



