188 Apple Canker Ffmrjus — Leaf Scar Infection 



growth as in June. When a leaf falls, the tissue exposed is submitted to 

 the desiccating effects of the atmosphere and results in the drying up of 

 the cells exposed. The contraction of the tissue which takes place on 

 drying results in small cracks appearing in the leaf scars, especially in the 

 region of the soft tissue adjacent to the leaf traces. These small cracks 

 allow the canker fungus the opportunity to enter the host, which it does 

 very readily, before the host has had time to form a phellogen (Text-fig. 1). 

 The small depressions between the leaf scars and the main stem hold 

 up small quantities of water which probably aid the germination of the 

 spores of the fungus. The tissue of the leaf base is rather looser than the 

 normal cortex and the canker fungus grows very freely in the intercellular 



Text-fig. 1. Radial longitudinal section of a leaf base showing a crack in the leaf scar 

 and the mycelium about to enter the host. The intercellular space in the tissue below 

 the crack affords an unimpeded entrance to the fungus, x 34. m= mycelium; l.t. = 

 leaf trace; i.s. = intercellular space. 



spaces. The spread of the fungus is helped by the slow response of the 

 host to form a limiting phellogen, and the stem soon becomes completely 

 girdled (Text-fig. 2). 



The second period of infection, which takes place when the trees 

 become active in the spring, however, is very similar in its symptoms to 

 the autumn infection. Many tiny cracks in the leaf scar tissue are found 

 especially towards March and April when the buds are bursting. Through- 

 out the winter the buds gradually swell by normal growth, but the 

 swelling is usually sufficiently slow to allow the continuity of the phel- 

 logen to be maintained. The growth cracks, however, frequently extend 

 to considerable depths. With the enormous increase in growth in early 



