The canker, as already observed, attends especially the old age of some fruit trees, and of these the apple is most remarkably a sufferer. "I do not mean," says Mr. Knight, "to assert that there ever was a time when an apple-tree did not canker on unfavourable soils, or that highly cultivated varieties were not more subject to the disease than others, where the soil did not suit them. But I assert from my own experience and observation within the last twenty years, that this disease becomes progressively more fatal to each variety, as the age of that variety beyond a certain period increases; that if an old worn-out orchard be planted with fruit trees, the varieties of the apple, which I have found in the catalogues of the middle of the seventeenth century, are unproductive of fruit, and in a state of debility and decay".

Among the individuals particularly liable to be infected, are those which have been marked by an excessively vigorous growth in their early years. I had one in my garden at Great Totham, which for the first twelve years of its existence was remarkable for the unnaturally large size and abundance of its annual shoots. It then became grievously affected by canker, which at length destroyed it.

Trees injudiciously pruned or growing upon an ungenial soil, are more frequently attacked than those advancing under contrary circumstances. The oldest trees are always the first attacked of those similarly cultivated. The golden pippin, the oldest existing variety of the apple, is more frequently and more seriously attacked than any other. The soil has a very considerable influence in inducing the disease. If the subsoil be a ferruginous gravel, or if it is not well drained, and the soil be aluminous, and effective means are not adopted to free it of superabundant moisture, the canker, under any one of these circumstances, is almost certain to make its appearance amongst the trees they sustain, however young and vigorous they were when first planted.

How inductive of this disease is a wet retentive subsoil, if the roots penetrate it, appears from the statement of Mr. Watts, gardener to R. G. Russell, Esq., of Chequers Court, in Buckinghamshire. - A border beneath a south wall had a soil three feet and a half in depth, apparently of the most fertile staple, twice re-made under the direction of the late Mr. Lee, of the Vineyard, Hammersmith. In this the trees, peaches and nectarines, flourish for the next three or four years after they are planted, but are then rapidly destroyed by the canker and gum. The subsoil is a stiff sour clay, nearly approaching to a brick earth; and the disease occurs as soon as it is reached by the roots of the tree. But this is certainly not a conclusion warranted by the premises, because the acridity of the sap, whatever may be its source, would be likely to injure and corrode, in the first instance, those parts where the vessels are most weak and tender; now these, past dispute, are in the branches. Moreover, we generally see the youngest branches the earliest sufferers.

Pruning has a powerful influence in preventing the occurrence of the canker. I remember a standard russet apple-tree of not more than twenty years' growth, with a redundancy of ill-arranged branches, that was excessively attacked by this disease. I had two of its three main branches, and the laterals of that remaining, carefully thinned; all the infected parts being at the same time removed. The result was a total cure. The branches were annually regulated, and for six years the disease never re-appeared. At the end of that time the tree had to be removed, as the ground it stood upon was required for another purpose. John Williams, Esq., of Pitmaston, from long experience concludes, that the golden pippin and other apples may be preserved from this disease, by pruning away every year that part of each shoot which is not perfectly ripened. By pursuing this method for six years, he brought a dwarf golden pippin tree to be as vigorous and as free from canker as any new variety.

All these facts unite in assuring us that the canker arises from the tree's weakness, from a deficiency in its vital energy, and consequent inability to imbibe and elaborate the nourishment necessary to sustain its frame in vigour, and much less to supply the healthy development of new parts. It matters not whether its energy be broken down by an unnatural rapidity of growth, by a disproportioned excess of branches oyer the mass of roots, by old age, or by the disorganization of the roots in an ungenial soil; they render the tree incapable of extracting sufficient nourishment from the soil, consequently incapable of developing a sufficient foliage; and therefore unable to digest and elaborate even the scanty sap that is supplied to them. The reason of the sap becoming unnaturally saline, appears to be, that in proportion as the vigour of any vegetable declines, it loses the power of selecting by its roots the nourishment congenial to its nature. An ungenial soil would have a debilitating influence upon the roots, in a proportionate though less violent degree than a corrosive poison; and as these consequently would absorb soluble bodies more freely, and without that discrimination so absolutely necessary for a healthy vegetation, so the other most essential organs of nutrition, the leaves of the weakened plants, would promote and accelerate the disease.

These, reduced in number and size, do not properly elaborate the sap; and I have always found that under such circumstances these stunted organs exhale the aqueous particles of the sap very abundantly, whilst their power of absorption is greatly reduced. The sap thus deficient in quantity and increased in acridity seems to corrode and affect the vascular system of the tree in the manner already described. These facts afford us most important guides in attaining the desired objects, the prevention and cure of the disease. If super-luxuriance threaten its introduction, the best remedy is for the cultivator to remove one of the main roots of the tree, and to be particularly careful not to add any fertile addition to the soil within their range. On the contrary, it will be well, if the continued exuberant growth shows the necessity, for the staple of the soil to be reduced in fertility, by the admixture of one less fertile, or even of drift sand. If there be an excess of branches, the saw and the pruning knife must be gradually applied.

It must be only a tree of very weak vital powers, such as is the golden pippin, that will bear the general cutting of the annual shoots ns pursued by Mr. Williams; a new vigorous variety-would exhaust itself the following year in the production of fresh wood. No-thing beyond a general rule for the pruning can be laid down, and it amounts to no more than the direction to keep a considerable vacancy between every branch, both above and beneath it, and especially to provide that not. even two twigs shall chafe against each other. The greater the intensity of light, and the freer the circulation of air amongst the foliage of the tree, the better the chance for its healthy vegetation. If the disease being in a fruit tree be a consequence of old age, it is probably a premature senility induced by injudicious management, for very few of our varieties are of an age that insure to them decrepitude. I have never yet known a tree, unless it was in the last stage of decay, that could not be recovered by giving it more air and light, by careful heading in pruning, improvement of the soil, and cleansing the bark.