This section is from the book "Commercial Gardening Vol3", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
This disease is much more prevalent now than in past times; reasons for this will be given later.
As a rule it is popularly believed that canker is the result of a tree growing under unfavourable conditions. This is not correct, the disease is directly due to the action of a fungus, but at the same time the fungus may be greatly assisted in its work of destruction by the tree being weakly, due to bad cultivation or other causes. The symptoms of the presence of canker, as the name denotes, are a rugged or cankered appearance of the bark, which often commences in the fork of a branch. When first attacked, the bark usually shows several cracks arranged in a concentric manner, and as the bark is killed by the fungus it becomes dry and falls away, exposing the wood, which is also eaten away. A callus forms at the edge of the wound, which in turn is attacked by the fungus and killed here and there, producing a rugged mass round the edge of the wound, which continues to increase in size year by year. If the bark or callus near the edge of a canker wound is carefully examined with a magnifying glass, clusters of minute red bodies (fig. 349) smaller than the head of a small pin will be seen. These are the fruits of the fungus. When small branches are attacked, the fungus often completely destroys the bark, and eats into the wood to such an extent that the branch breaks at the injured point.
Fig. 349. - Apple-tree Canker (Nectria ditissbna).
Branches of an apple tree showing the bark destroyed by the fungus. The little white points in the cracks on the diseased parts are the fruits of the fungus, which are of a bright-red colour (natural size). (Gard. Chron).
The Nectria is a wound parasite; that is, it can only gain an entrance into the tree through some wound, which need not necessarily be very large. Wounds in the fork of a branch are often caused by the wind twisting the branch and causing a slight cracking of the bark where the branch springs from the trunk, or by the branch being pressed down by snow. Into such minute cracks the spores find their way, and set up the disease. When canker appears on the smaller branches it almost invariably arises in those places where the bark has been injured by the Woolly Aphis, or American Blight, in fact it may be truly stated that canker, as a serious disease, appeared for the first time after the general spread of American Blight. The Nectria attacks various other trees, as Beech, Oak, Hazel, Hornbeam, Maple, etc, but in no instance has it increased to the extent of becoming a serious epidemic, except on the Apple tree, because there alone it is aided by the wounds made by American Blight.
Where the disease has gained much headway the branches should be removed, as the mycelium of the fungus spreads in the wood considerably beyond the wound. In the case of slight wounds the diseased portion should be cut away, going well beyond the boundary of the wound; the scar should at once be painted over with a coating of gas tar to insure against further infection. Methods should be adopted for the destruction of American Blight, care being taken to remember that this pest frequently hibernates on the underground part of the trunk, or the collar, during the winter months. [g. m.]
"Hawthorndens" will canker as if smitten with leprosy on thin, poor, chalky soil, but will maintain a healthy vigour for years upon a generous soil. When once an Apple is attacked badly with this fungus it is a difficult matter to get rid of it. Probably by the time this occurs some other sorts on the plantation have given evidence that they are happy, and if they or any of them also crop well, and the fruit sells well, the cheapest course in the long run may be to do away with the cankered variety and substitute one that has proved its suitability to the soil and situation. The late Mr. T. Rivers gave a formula of manures to cure canker; it was quoted by Mr. Wise in his paper before the Royal Horticultural Society already referred to. One should receive with the greatest respect any advice backed by the exceptional knowledge and experience which was possessed by the late Mr. Rivers, but one may doubt whether any given combination of manures can be expected to be applicable to every soil, or to meet the difficulties of every variety of Apple that finds itself unable to resist the attacks of the fungus. When the long-talked-of Government experiments in fruit growing are inaugurated, and if by chance it should happen that they were practically managed, it may be possible to discover what it is that is out of alignment between certain soils and certain varieties of Apple that reduces the vitality of the tree so as to place it at the mercy of the canker fungus.
At present, from such observations as are possible to a non-scientist, it seems that certain varieties of Apple will do well in soil A, and canker in soil B, while others will do well in soil B, and canker in soil A.
In order to check the spread of the spores, and to give the tree a chance of overcoming the attack, a good plan is to spray in the late winter, just before the buds begin to burst, with a solution of sulphate of copper, 1 lb. to 36 gall, of water; the spray should be driven hard into the canker sores.
[w. G. L.]