This is perhaps the most dreaded of all fruit enemies. It is no respecter of kinds or varieties Under certain conditions it will set its evil clasp on almost everything; in other circumstances it will limit its attack to a few sorts. It does the most damage amongst Apples; and of varieties, perhaps there are no worse sufferers than Ribston Pippin and Lord Suffield. It is important to remember that a variety which is very little affected in some districts is very badly attacked in another, where the soil is different - probably heavier. As an instance, Lord Suffield thrives in many parts of Kent, notably in the Swanley district. Yet I know an orchard on clay in East Kent in which every tree is cankering to death. The culture is good, and other sorts succeed, but Lord Suffield is a rank failure. Instances of this sort are instructive, for they show what a number of side issues there are in fruit growing. I shall give special attention to this matter when I come to make my selections.
What is canker? It is a fungus, and its name is Nectria ditissima. Does it pierce healthy bark, extract juices from the sap vessels, and so cripple the tree? No, it waits until it finds a joint loose in the bark armour, then thrusts in its germinal tube, and establishes itself. It spreads under the bark, and the latter becomes unhealthy. An experienced eye can tell when a tree is "going to canker" - as a matter of fact, it is already cankered - by the appearance of the bark, which becomes swollen and chippy some time before a large* wound is seen. The fungus throws up propagating growths in autumn and early winter, and the faster it increases the more unhealthy the bark becomes. The wound d nearly halfway up the tree A in the figure is a typical one. A great deal of bark has died away altogether, leaving the smooth inner wood exposed; there is also dead bark.
Usually the latter is associated with rolls of clean, healthy-looking growth. Good and bad influences are at work here. The fungus is attempting to girdle the stem so that it may die; the tree is endeavouring to cover the exposed area with new tissue so that it may live.
A, part of the stem and branches of an Apple tree: a, portions of clean stem; b, an affected branch; c, diseased branches.
Canker on stem: d, wound with corticated baric surrounding the wound, but not giving visible sign of infection by parasitic organism; e, wound bearing in clefts of the bark at the circumference of the wound fruits of Canker Fungus, Nectria ditissima.
Canker on branches: f, completely girdled, and parts above the wounds killed, young growths pushed from healthy parts below points of attack, but the parasite is still existent in the bark below the dead parts of the branches; g, wounds on branches, but not to the extent of girdling them; h, shoot killed in the year of attack.
B, fruits (perithecia or spore capsules) enlarged 25 diameters.
C, asci or bladders of Fungus with spores, magnified 250 times.
D, spore of Fungus: i, germinal tube, enlarged 650 diameters.
Higher up, at e, is seen another wound with the canker fruits showing, and at various points on the side branches other wounds may be seen.
It will be well to glance at a few causes of canker before coming to remedies. In my opinion there are three which eclipse all others, and I will place them in what I consider to be their order of infamy:-
1. Unsuitable soil, either from poverty or peculiarity of variety.
2. Predisposing injury by aphides, birds, or man.
3. Rubbing by crossed branches.
If the soil in which fruit trees are grown were systematically cultivated and manured (see previous chapter), there would be two-thirds less canker than there is now; and if growers were careful in choosing varieties the proportion of unhealthy trees would be still less.
It is impossible to prevent all external injury to fruit trees, but by keeping down aphides and exercising care in handling at planting and pruning time the damage is reduced to a minimum. It is quite possible to avoid all injury by branch-rubbing (see "Pruning").
Can canker be cured when it has got a firm hold? This is a question that is usually answered in the negative. Personally, I protest against so swift a surrender of the position. It is easy, of course, to "let things slide," but it is not very profitable, nor altogether creditable either. There are doubtless cases in which the enemy is master of the situation, but there are certainly a great many others in which he could be met and defeated.
If a fruit grower has a number of trees which canker badly, and they prove to be of one or two particular varieties, he will be justified in concluding that the soil does not suit those sorts. If the trees are old the best thing will be to get rid of them; if they are young he may graft them with a stronger sort (see chapter on "Grafting"). In Nottinghamshire I have seen trees of Ribston Pippin, so badly cankered that the trunk was nearly girdled, brought back to health and vigour by grafting them with the strong and hardy variety Bramley'? Seedling. The great gaping wounds on the bole have completely healed.
Where canker is pretty general, affecting nearly all the varieties, it is safe to assume that there is something wrong with the soil. It is perhaps undrained; or it is poverty stricken. Want of drainage accounts for an enormous amount of loss from canker. Fruit plantations on a slope are usually drained naturally; but on a plateau, or in a valley, pipes are frequently wanted. If pipes are laid 30 inches deep in strong land, and the soil above is bastard trenched and winter ridged, the ground will be so much warmer, sweeter, and more fertile that the trees are bound to be benefited. If the soil is poor it should be fed in the way described in a previous chapter. I have often known so simple a thing as a coat of manure check canker.
In the early stages of the disease much good may often be effected by cutting out the diseased portions with a sharp knife or chisel, and then dressing with Stockholm tar. I have thus operated on hundreds of trees, and while on the one hand I have never seen a solitary instance of injury accrue, I have in many cases known direct benefit follow. Every particle of diseased and unwholesome-looking tissue should be cut out, and the Stockholm tar (which may be purchased quite cheaply at any gasworks) applied with an ordinary paint brush.