186 Ajyple Canker Fungus — Leaf Scar Infection 



In addition to infection of the recently formed leaf scars, occasional 

 infections of a stem two or three years old take place in the region of a 

 dormant bud. Such infections are readily distinguishable, at least in 

 their early stages, by their similarity in appearance to the infections on 

 the young wood, by the lack of any swelling of the stem and the absence 

 of concentrically arranged fissures which are so characteristic of cankers 

 of long standing. 



STAGES IN THE DEVELOPMENT OF LEAF SCAR INFECTION. 



In order to find out how the infection is brought about, search was 

 made for the early stages of bud infection. 



The seat of infection appears to be associated with the base of the 

 bud rather than the interior of the bud itself. The earhest stages of in- 

 fection are sometimes not visible externally at all and the bud has a 

 normal appearance. It is only on cutting the stem that the infected 

 tissues can be discovered and in many cases it is found that a small 

 lateral portion of the leaf scar is infected, although the colour of the bark 

 and the appearance of the bud are perfectly normal. 



The first visible external signs of any disorder varies in different 

 cases, but is usually associated with a small crack in the leaf scar. The 

 sap of the apple stem quickly oxidises to a bright reddish brown colour 

 on exposure to air and this property of the sap enables small injuries to 

 the leaf scar tissue to be detected fairly readily. On strongly grown 

 shoots, such as the "leaders" springing from the crown of the trees, no 

 visible spUt develops at all, the first stage of infection being seen in the 

 formation of a circular dark reddish brown spot on the main stem at the 

 extreme edge of the leaf scar. This spot, at first the size of a pin's head, 

 increases rapidly to about the size of a pea. 



The next definite stage in infection is the development of the primary 

 scar, i.e. the scar first hmited by the newly formed phellogen. This 

 development arises rapidly and varies in position according to the exact 

 spot where infection first took place; it is usual, however, to find it 

 rather below the leaf scar and more often in a lateral position than a 

 median one (Plate III, Fig. 1). When once the fungus has entered the 

 stem, growth is extremely rapid and it is only a short time before the stem 

 is completely encircled. The tissues of the bud being young and succulent 

 are very quickly attacked by the fungus, so quickly indeed that it is often 

 difficult to obtain a young leaf scar infection in which the bud itseK is not 

 infected with mycehum. The separation of the cuticularised layers of the 

 bark from the underlying cortex frequently occurs, and these form a 



