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The trunk of fruit trees can be damaged by various means. If the bark is removed the tissues that transport food manufactured in the leaves to the roots are also removed. The root system as a result has no source of food and will slowly starve. The above ground portions of the tree will gradually decline and eventually die as the root system uses up all reserves of energy and finally fails.
The bridge graft is a method of repairing a girdled trunk. Although it can be done by inserting the bridges into cuts made in the wood of the trunk, the common method is to lift the bark and place the exposed cambiums of scion and stock together. It is thus usually a bark graft and is not done until May. In this case it is necessary to collect the dormant scions earlier and store them until the repairs are to be made. Healthy, matured suckers of hardy varieties are suitable.
Where one-quarter or less of the trunk circumference has been girdled, the necessity of bridge grafting may be doubtful, but a wound dressing applied in early spring is always helpful, even to these less extensive injuries.
Trees which have been in the orchard less than 4 years are usually too small for successful bridge grafting. If they are completely girdled, or nearly so, and there is still a collar of live bark above the graft union, saw the top off at a point immediately below the injury. Apple, pear, and plum will develop a new top without grafting. Cherry and peach are unlikely to do so; and require grafting of scions on the remaining trunks, or preferably the planting of a new tree. Apple, pear, and plum, when cut back, will produce many shoots. These shoots should not be thinned until they are a year old, then only the most suitable ones should be retained.
Of the numerous methods of bridge grafting, the channel or inlay method is generally preferred (Figure 1). The two possible locations of the channel depend on whether or not the wound has been treated previously with a protective covering. Where the wound has been covered some time before the grafting operation, the channels must be well above and below the treated area; otherwise the method shown in Figure 1 is more desirable, particularly with regard to speed of the operation.
Prepare the scions with the bevel on the side opposite the natural bow of the wood. When a scion has been cut to the proper length and bevelled for 5 to 8 cm at each end, it is laid over the wound in the position it is to occupy. By outlining the scions on the bark of the stock with the knife point, an almost perfect fit of the scions in the channels is possible. The distance between the extremities of the channels should be slightly less than the length of the scion, allowing for a slight bow of the scion when the job is completed. This slight bend allows better contact of scion and stock and reduces the danger of breaking connections if the tree sways with the wind.
With a screwdriver, lift and remove the strips of bark from the channels. If these strips do not come away readily from the wood, postpone the work until they do. Place the lower (thicker) end of the scion in the bottom channel and nail it there with two 2.5 cm box or basket nails. Then spring the upper end into position and nail it similarly. Place scions about 5 cm apart over the injured area and then cover all wounded surfaces thoroughly with a good quality grafting compound that will not shrink or crack.
If the girdled tree has one or more suckers growing from its root, these suckers can be grafted into the trunk (or branches) above the wound. The operation is called inarching.
Where no suckers exist, an extensive trunk wound, too long or extending too low into the roots for ordinary bridge grafting, can be overcome by planting one or more nursery trees around and close to the trunk of the orchard tree. There should be one for each limb on the injured tree. The upper ends of these trees are cut and inlaid into the trunk or main limbs above the wound in the same manner as described for bridge grafting (Figure 2).
One cannot expect maximum success if the trees are neglected after grafting. Two most common reasons for graft failure are: putting the scion in upside down and, allowing the scion to dry out by improper application of the grafting compound. Once or twice in the few weeks following grafting, examine the wounds. Whenever the covering has opened, reapply the grafting compound. It is not necessary to remove leaves that may form the first year on the scion. These leaves may help to strengthen the scion during the healing process.
With some fruits there is a danger that virus symptoms may appear following grafting. The growth of scions should be watched closely. If any unusual symptoms are noticed, the grower might try topworking another variety. However, the removal of the trees would be preferable. A professional opinion should be sought regarding the symptoms.
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