10 



varieties of apple. In many cases the wound is confined to 

 one side of the branch and the branch continues to live for 

 many years. In other cases the parasite " rings," that is com- 

 pletely encircles, the branch or shoot, as shown in Fig. 2, 

 and causes its death. Provided the branch is not ringed the 

 sap continues to flow, and such branches remain alive and 

 may even grow and yield heavy crops of small fruit. 



The fruiting bodies of the fungus, by which alone the 

 disease can be identified with absolute certainty, are 

 minute and occur especially in autumn and spring. 

 Those formed in spring and early summer are perhaps 

 the best known. They occur on various parts of 

 the wound and consist of round, rather tough bodies 

 (perithecia), deep crimson in colour and about the size of 

 a poppy seed. Large numbers of two-celled spores are 

 liberated from these fruit-bodies and are conveyed by rain, 

 wind, insects, &c., to other trees, and thus spread the disease. 

 In late summer and autumn especially the fungus develops 

 another form of fruit, which shows as white, very minute 

 spots. The fruiting bodies bear masses of sickle-shaped spores 

 (conidia) and these also serve to spread the disease. 



Causes Leading to Attack. The canker fungus gains 

 entrance to a tree by means of wounds or small injuries, and 

 apart from such wounds the fungus spores are not able to 

 infect the branches. The wounds may be large, such as 

 those caused by breakages, careless pruning, Ac., but minute 

 injuries, especially those occurring naturally in the region 

 of the buds, provide a very efficient means of entry. 

 Recent work at Long Ashton shows that the leaf-scars left 

 by the fall of the leaves in the autumn afford an excellent 

 opportunity for the fungus to enter the tree, and, during 

 the winter and following spring, examples of this type of 

 infection 6f the previous year's shoots can be found in almost 

 every orchard. Other wounds which may frequently lead to 

 canker are frost cracks and the breaking of young shoots due 

 to careless picking or pruning. The " heading back " of 

 badly grown trees exposes large cut surfaces to infection, and 

 main branches of trees treated, in this manner are often found 

 to be severely attacked by canker. 



Another, and very important, means of entry is through 

 injuries caused by Woolly Aphis. The soft swollen tissue 

 produced by the Aphis is very apt to become cracked' during 

 the autumn, exposing the woody tissues of the stem. The 

 fungus quickly infects such splits, and infections of this type 

 are quite commonly found on the young wood during the 

 winter. One other method by which the canker fungus 

 attacks the tree must be mentioned. The scab fungus towards 

 the autumn attacks the young shoots formed during the 

 summer, resulting in the formation of numerous scars on the 



