OBSERVATIONS ON APPLE CANKER III. THE ANATOMY OF THE STEM CANKER

The development of cankers caused by Nectria galligena on apple is described; the pathogen exploits all the tissues outside the xylem and will also penetrate the xylem to an appreciable depth, invading the xylem parenchyma, vessels and fibres. Spread in the peripheral tissues is checked to some extent by successive barriers laid down by wound phellogens, but the barriers are eventually passed by the mycelium. There appear to be two distinct ways in which these barriers are passed: there may be direct penetration which appears to be associated with an aggregation of the mycelium into blocks, and symptoms suggestive of mechanical rupture; or there is an alternative route in which the pathogen grows in the lumen of the fibres and emerges behind the barrier. In the xylem the spread of the pathogen is checked by tyloses and gumming in the vessels, and by gumming in the parenchymatous tissues, but in the fibres there appears to be no defence mechanism, and the spread beyond the gum barriers was usually recorded in the xylem fibres. It is suggested that the presence of the pathogen in the xylem fibres might provide the explanation of the formation of cankers on partially healed pruning cuts, and this type of canker is described in some detail. The development of the canker depends on the balance between the development of the pathogen and the resistance of the host, and there is some discussion on the relation of this balance to the control of the disease.