280 St^idies on the Apjjle Canker Fungus 



pass a well-formed cork layer, but if no such barrier exists or if it is only 

 partially developed then the fungus can progress, apparently by the help 

 of its secretions. 



The later stages of infection. Once the canker fungus has effected an 

 entrance to the cortex, it proceeds to grow very rapidly in all directions 

 chiefly in the intercellular spaces. Concurrently with the gradual progress 

 of the infection by the fungus the healthy cortical tissue usually becomes 

 very active, its cells dividing rapidly and the intercellular spaces being 

 more or less obhterated. This rapidly dividing tissue unless protected by 

 a cork layer soon becomes infected with the canker mycelium, and under- 

 goes changes described above. The host plant persists in its efforts to 

 form a wound cork layer especially in the region between the scleren- 

 chymatous bundles of the cortex, apparently to prevent the fungus from 

 entering the wood. Sometimes the growth stimulus of the cells of the 

 developing phellogen layer is so strong that the cells hypertrophy and 

 the whole tissue becomes ruptured at this region. When the infection 

 has penetrated too deeply to be excluded by a cork layer and cannot be 

 prevented from reaching the wood, wound wood is formed, consisting of 

 medullary ray-like cells, and these offer considerably more resistance to 

 the path of the fungus than the vessels, since the contents of the brick- 

 shaped cells of the wound wood become choked up with gummy material 

 which especially collects at the pits through which the hyphae of Nectria 

 normally pass. Sometimes the fungus reaches the wood before any wound 

 wood can be formed and in this case wound wood is cut off from the 

 cambium which still remains living round the infected area. In the cortex 

 a strong cork layer is usually formed ultimately round the infected tissue 

 and this has the effect of limiting the infection and is largely responsible 

 for the concentric cracks m the canker scar which are so characteristic 

 of the disease. Not infrequently the stem becomes completely girdled 

 and the whole of the shoot above is killed off. 



This manner of infection appears to be unusual amongst fungal para- 

 sites. Fungi parasitic on other fungi are known, but for one, unable to 

 penetrate uninjured bark itself, to take advantage of the injury effected 

 by another and subsequently supersede the latter is unique. 



Control. 



The obvious way of controlling the infection of scab wounds by 

 Nectria is to control the autumn infection of the scab fungus on the wood. 

 It is the usual practice to spray against scab in the spring to protect the 

 fruit and no measures beyond cutting out diseased wood are taken 



