In the removal of cankered limbs the cut may be made so that another shoot can grow in the approximate space left by the portion removed. The position of the canker on the limb determines the feasibility of the operation. In case the canker is at some point a few feet from the parent branch the affected portion of the limb may be removed by sawing off the same between the canker and the parent branch. On the cut end may then be grafted a desirable variety. This grafting of stubs is justified in certain cases, but should always be supplemented by careful spraying of the bark. The grafts should be watched closely, since the canker fungus may enter the limb at such points.
In cases where the whole limb is cut from the tree, the cut should be made at the junction of the limb with the parent branch. The cut should be made close to and perfectly even with the outline of the larger supporting limb without regard to the size of the resulting wound. A rough, splintered surface is highly undesirable and may be avoided by the following procedure preliminary to making the final cut. Saw about one-third through the limb from the lower side at a point one foot beyond the place where the final cut is to be made. The next cut on the upper side is made an inch beyond the first cut. The cutting on the upper side should continue until the limb is severed. This leaves a stub about a foot long which must now be removed, cutting, as previously outlined, close to the main limb. It is important that no stub be left, even if it is but a few inches in length, for the presence of a stub retards, or wholly prevents, healing. In those cases where a callus does not occlude the wound in a relatively short time, Heart Rot is almost sure to result. It requires comparatively little time and expense to clean and protect a fresh injury or decay in its early stages. It often necessitates the expenditure of a great deal of time and money to treat properly the same injury after it has been neglected for a few years.
The cutting - out of cankers is a method to be employed when the orchardist is satisfied that the value of the limb warrants it. An attempt to remove all kinds and sizes of cankers from an infested orchard, without regarding the value of the limbs, is likely to result in discouragement with the whole matter. Such, practice is neither good nor profitable. The grower's judgment must guide him.
Certain tools have been found advantageous for this work. A draw-shave for use in removing diseased bark, and a farrier's knife for trimming the margin of the wound, are the chief tools needed. The knife must be sharp, for a dull edge may injure the growing part. The pruner should not wear heavy leather-soled shoes, since canker fungi may get into the bark through ' wounds caused by such shoes. It is suggested that rubber boots, or some type of soft - soled shoes, be worn in connection with such operations.
In treating cankers it is necessary to determine the limits of the diseased tissue. This may be done by examining the canker externally or by shaving off bits of bark until the line of discoloration is located. The depth of the cut depends on the depth of any indication of disease, that is, discoloration. If the canker is for the most part superficial, penetrating the wood only in spots, the bark may be removed as described above, the deeper spots being rimmed out with a farrier's knife. If the fungus has entered the wood, either locally or in long streaks, the discolored part must be removed. If the streak extends for a considerable distance, the case may warrant the removal of the whole limb. So far as possible the wound when finally shaped should be pointed above and below, as this facilitates healing; if the cut is left in a rectangular form, the upper and lower edges heal more slowly. The edge of the wound should be cut at right angles to the surface of the bark; cuts made otherwise will result in a certain amount of dead bark, which makes an easy entrance for canker fungi.
It is a common, but false, belief that some substance may be applied to the surface of a wound to accelerate healing. No wound-dressing can induce more rapid callus formation, but it has been found very desirable and helpful to split the edge of the callus each spring, next to the wood, in order to stimulate wide spreading of the callus. The rapidity of the healing process depends on the character and position of the wound and the time of year when the wound is made, rather than on protective coverings. The sole object of coating a cut surface is to protect the heart - wood from decay until the new growth, which forms from the growing tissue immediately beneath the bark, has had time to develop over the exposed dead wood and protect it from decay. The fundamental requirements of a wound dressing, then, are that it be a preservative and a preventive. It should have antiseptic qualities, and should be fluid, reasonably inexpensive, and easily prepared and applied; it is essential that it give a complete covering; it must be impervious to air and water, must be durable, and must not injure nor kill the tissues nor interfere in any way with the healing process.
Preparations that meet all these requirements are not to be obtained. The substances most commonly used are paint, asphaltum and tars. Paint is an inefficient covering; asphal - tum, once applied, gives good protection, but it is more difficult to prepare and to apply, especially when it is liquefied by heat. Asphaltum is more readily available if it is dissolved in gasoline. This combination has been used on apples with satisfactory results.
The writers have used coal-tar for the past three years with good success. This is produced in the manufacture of artificial gas from coal. It should not be called gas-tar, which is a term more loosely used. Some growers have complained of injury to trees from the use of coal-tar, but in such cases the material has usually proved to be something other than coal - tar.
The effectiveness of spraying for canker is a question frequently raised. As a preventive this operation is worthy of attention; as a cure it is out of the question, for once the fungus is in the bark the spray material cannot reach it.
If an orchardist desires to grow susceptible varieties, the canker menace may be obviated to a considerable extent by working over the larger limbs of more resistant varieties to the one desired. This has been done with apparent success in a few instances. The difficulties involved are that pruning must be done every year in order to remove all sprouts from the stocks, and that the renewal of old branches cannot be effected so rapidly. Such treatment also throws the bearing area higher in the air; so that in the case of erect - growing varieties such as the Twenty Ounce, it makes pruning, spraying and picking more difficult.
For the control of Black Rot of apples, bordeaux mixture is said to be more effective than lime sulfur. Apply bordeaux 4-4-50 as follows: (1) about July 15, when the disease is just appearing, and (2) two weeks later. As a matter of precaution against damage in storage, avoid injuring the fruit in any way. Spraying for Leaf Spot should be done according to the schedule outlined for apple scab (see page 12).
Hesler, L. R. Black rot, leaf spot, and canker of pomaceous fruits.
Cornell Univ. Agr. Exp. Sta. Bul. 379: 51 - 148. 1916. Hesler, Lex R. Apple cankers and their control. Cornell Univ. Agr.
Exp. Sta. Circ. 28: 17-28. 1915. Scott, W. M., and Rorer, J. B. Apple leaf spot caused by Sphaeropsis Malorum. U. S. Agr. Dept. Plant Indus. Bur. Bul. 121:47 - 53.
Paddock W. The New York apple tree canker. New York (Geneva)
Agr. Exp. Sta. Bul. 163: 179-206. 1899. Paddock, W. The New York apple tree canker (second report). New York (Geneva) Agr. Exp. Sta. Bul. 185: 205 - 213. 1900. Brooks, Chas., and De Meritt, M. Apple leaf spot. Phytopath. 2:
181-190. 1912. Wolf, F. A. Control of apple Black Rot. Phytopath. 3:288 - 289.
1913. Hesler, Lex R. Physalospora Cydoniae. Phytopath. 3: 290 - 295.
1913. Lewis, I. M. Apple leaf spot. New Hampshire Agr. Exp. Sta. Repts.
19 and 20: 365 - 369. 1908. Hesler, Lex R. The New York apple tree canker. Indiana Acad.
Sci. Proc. 1911: 325 - 339. 1912.