276 SfiffUes on the Aiyple Canker Fungus 



and diseased tissue and the little scab infection can often be identified 

 in the middle of the scar. In the autumn and winter canker infections of 

 scab wounds are thus most. frequently found in this stage (see Fig. 2). 

 Later stages of development are often characterised by the forma- 

 tion of well-defined cracks at the edge of the infected area and a slight 

 swelling of the adjacent tissue (see Figs. 3 and 4). If the tree is suffi- 

 ciently vigorous to form a cork layer round such a scar before the wood 

 has become infected, the canker makes very little progress, and the tree 

 makes a good fight against the fungus. Often, however, the whole of 

 the cortex becomes infected and the fungus reaches the woody tissues. 

 In these cases, examples of which are seen in Figs. 5 and 6, the scar is 

 more like a normal canker produced by the canker fungus. It is some- 

 what difficult to assign any particular method of infection to a mature 

 canker. The presence of the concentric cracks in the bark, however, 

 localises the original point of infection and the appearance of this spot 

 is sometimes strongly suggestive of infection through a scab wound. 

 Fig. 7 is a photograph of such a case and other instances have been 

 found on the pear as well as the apple. Fructifications of the fungus are 

 not borne until the canker is well developed, but on keeping young 

 infections in a moist chamber for two or three days, a few small conidial 

 pustules generally appear and afford evidence of the presence of the 

 canker fungus. 



The occurrence of this type of infection has not been found to be 

 nearly so common as that of the leaf scar infection previously described 

 but it is probably as prevalent as the infection which takes place through 

 woolly aphis galls. In some years when the autumn has been specially 

 damp, the shoots of the previous winter are often found to be killed off 

 in large numbers. Such shoots are usually heavily infected with scab 

 and although they frequently bear leaf scar infections of the canker 

 fungus, it is probable that canker infections of scab wounds are respon- 

 sible for a good proportion of the damage. 



Microscopic Details. 



The establishment of the canker mycelium upon the scab stroma. Early 

 in the autumn, when the scab pustules are very small and before any 

 really definite signs of infection can be observed by the naked eye, 

 microscopical examination has revealed the presence of Neclria galligena 

 in a number of instances. The minute cracks which occur in the bark 

 covering the young scab infection afford a favourable lodgment for the 

 conidia of the canker fungus, which germinate readily under the moist 



