American Chestnut Cooperators' Foundation

Grafting For Blight Resistance

Scions grafted into established root systems can reach maturity, producing nuts, within 3 to 4 years, as opposed to the 7 to 10 year interval to be expected from seednut to nut-producing tree.

 ACCF grafting scions are clones of American chestnuts with tested blight resistance. They are cut in January from the previous season's growth on resistant American chestnuts, and stored in plastic bags  in the referigerator until grafting time, (when leaves emerge and the bark is slipping). We bring scions to the grafting site in a cooler to keep them fresh.
 

J.Bruce Given was the Chestnut Project Leader for the West Virginia Department of Agriculture from 1976 until his death in 1985. Bruce pioneered this grafting technique for propagating scions of old survivors. His work facilitated establishment of American Chestnut Cooperators' Foundation breeding orchards. This bark graft (incorporating information from Hongwen Huang, who identified a major cause for chestnut graft failure in the phloem fiber bundles) is taught by John Rush Elkins each May, at Bruce Given Memorial Grafting Clinics.

 PREPARATIONS

By April, American chestnut stems to be used as grafting stock should be flagged and fertilized. On forest sites, clear an area 10 feet in diameter around each chestnut stem and fertilize with one cup of diammonium phosphate (18-46-0), if available, per inch of stem DBH. On nursery sites, fertilize with one cup ammonium nitrate plus one tablespoon zinc sulfate, or substitute, per inch of stem DBH. Fertilizer should be applied in a circle at the drip line or at least 2 feet out from the trunk. Choose stems which are blight free at the base, so that you have healthy bark to accept the grafts; choose stems 1 to 2 inches in diameter at the base, so that short tree shelters will fit over them.

 Ten days to two weeks before grafting, cut the flagged chestnut stems 2 feet from ground level and prune off any lower branches, so that the rising sap will not push out your grafts.

 BARK GRAFTING (when the sap is running and bark slips)

1. Using a sharp grafting knife, cut a segment of scion about one inch long with one bud at or near the top. On the side behind the bud, notch out halfway into the twig and straight downward.

 2. On the bud side, cut a wedge across the bottom end, so that one vertical side of the wedge is wider than the other. The surfaces of the vertical cut and the wedge should be perfectly flat.

 3. On the wide side of the wedge, very carefully peel away only the outer waxy layer of bark in one narrow strip, leaving the green exposed. Put the scion into a plastic bag containing water until you have prepared the stock to receive it.

4. Cut the prepared stump near to ground level, being careful not to peel the bark downward.

 5. Place the prepared scion with its notch at the top of the squared off stock against a smooth place on the bark to measure your cut. (Try to avoid cutting through phloem fiber bundles. If these structures are not visible, make the cut through a bud axil or graft 2 scions.) Place the blade along the broad side of the wedge and cut through the bark, angling slightly away from the scion, to provide a slot for the wide side of the wedge to fit. Hold the bark tightly against the stump as you withdraw the knife blade so that the side not to be raised is not loosened.

 6. With your blade, carefully lift the bark on the other side to form a flap. Insert the scion underneath the flap and down to the notch with the peeled edge lined up adjacent to your cut and the flat edge (from your first cut) flush with the wood.

 7. Secure with grafting tape wound around the circumference of the stump.

 8. Seal with grafting compound or a Swiss-brand grafting wax applied along the vertical cut in the stump and on the exposed surface of the stump, around the scion, and on the top of the scion (unless an apical bud was used). Be careful not to touch or cover the bud.

 9. Protect the graft from the elements by installing a short tree shelter (6" to 12" TUBEX) covered by a white paper bag. Mound up soil around the bottom of the shelter so that air cannot move up it, as in a chimney.

 10. Return 2 weeks later to check the grafted stems, pinch off suckers developing below successful grafts, and cover the stump with soil to protect the graft union from blight; remove the bag as soon as the graft begins to grow. Tie grafts loosely to the stake every few feet, and attach hair bags.

 SERIOUS PROBLEMS AND POSSIBLE SOLUTIONS

 1. Grafts lost in the woods. When making your trail, consider the rapid summer growth which might hide it and use markers that can be seen amidst dense foliage.

 2. Volunteer sprouts below or beside grafts cause failure, then they are mistaken for the grafts. Return at least every other week through July to cut out suckers; once a week is better.

 3. In heavy rains or long wet periods, shelters may hold too much moisture, drowning grafts or causing mildew to kill them. As soon as a graft has 3 or 4 leaves, remove its shelter, but leave it attached to the stake to give protection on the most exposed side.

 4. Grafts that are healthy in September are killed over the winter. In late July prune all grafts back by 1/2 to strengthen the graft union and lessen the weight it must bear through winter snow and ice storms.

 5. Soil protecting the graft union from blight is washed away, causing the graft to die during the first winter. In September mound up more soil, surrounded by rocks, branches or held in place with moss to prevent erosion.

 REPORT: Make a map, recording the location and genealogy of your grafted scions, so that you will be able to report to us each September: which grafts have survived, and their height when pruned; thereafter, please let us know annually the names and numbers of surviving grafts,  whether scions from them may be cut to share with other grafting cooperators within your area, and when your grafts develop blight  (for inoculation).

To discover how grafting fits into the ACCF strategy for American chestnut restoration in our forests, go to Managing Aging Clearcuts For American Chestnut Revival.

The following is a translation of recommendations published in IL DIVULGATORE for chestnut growers in the Province of Bologna and passed to us with the compliments of Dr. Tullio Turchetti, Forest Pathologist, University of Florence, by his student, Dr. George Maresi.

Please keep in mind that dates mentioned are appropriate for that Italian province; to learn the best times for executing the various grafts in your area, try a variety of dates and keep a book in which climate data as well as the various signs of the onset of spring are noted for each day that you graft.

 Also, we advise disregarding the Italian preference for grafting well above ground level, which is not appropriate in the U.S., because of the abundance here of virulent blight fungus, the greater susceptibility to blight of American stock as compared to European, and the probability that successful grafts would be killed by cankers forming on the stock below the graft. For these reasons we graft at ground level where the stock and the graft union can be protected from blight by mounding the soil up to cover them.

For successful grafting it is essential to minimize contamination of cut surfaces on the stock and scion by blight fungus. Therefore, it is necessary:
a) to disinfect repetitively, with 10% sodium hypochlorite (chlorine) or with a corrosive sublimate at 1 %, all the instruments required for grafting. (VT recommends disinfecting with Lysol, Listerine or rubbing alcohol, all less corrosive to metal tools.) A good rule would be to carry out this operation after each stump.
b) to cut the stock at a stable height for the graft just before the final operation.
c) to avoid peeling and all accidental cuts.
d) not to bind the graft too vigorously: excessively tight ties produce necrosis around the whole grafting area and favor the insurgence of blight.

 Particular attention must be paid not to touch with the fingers the generative tissues uncovered by cutting during the grafting operation, to avoid triggering processes which could hinder the take. Try to reduce to a minimum the time necessary for grafting, and prefer days with elevated humidity, no wind and temperatures fluctuating from 59 to 68 degrees F.

 The choices of grafting method are (in order of preference) the double triangle, whip, cleft and flute, made on stocks of small dimensions, maximum diameter of 3/4 in. The triangle, cleft and whip graft joints are less susceptible to blight, since their surfaces close rapidly, making a perfect weld between the scion and stock, also minimizing desiccation. In this respect, the bark graft, executed with 2 scions, on stocks about 2 in. in diameter, is more fragile and is employed mainly to obtain a great quantity of scionwood. The grafts made near soil level, give optimal results in attachment and vigor, but demand more frequent cleaning of the stump sprouts.

 BARK GRAFT:
This method, used predominantly in the chestnut culture region between Bologna & Rome, consists of insertion of the scion under the bark of the stock, after having cleaned up the surface of the cut with a grafting knife. This graft can be executed only when the bark is slipping, here (in Italy), beginning in the first half of April. When working on stumps of diameters 1.5 to 2 in., use 2 or 3 scions; for larger diameters, increase the number of scions. However, the larger number of scions necessary and longer time required for recovery of the surface of the cut favor a blight attack and the penetration of the microorganisms that cause wood decay.

 The scion should be cut in the part turned opposite the center of the stock with a single cut slightly oblique, otherwise with 2 cuts, to obtain a step which serves to support the scion. Usually, a thin strip of bark from the reverse external side of the scion is removed. Normally for this type of graft, short scions having 2 healthy buds are used, because scions of greater length could shift, causing failure of the graft. These are tied with raffia or other suitable material. The surface of the cut and the top of the scion are covered with plenty of grafting tar or wax to avoid dehydration.

WHIP GRAFT:
For this graft the scion and stock must have the same diameter, preferably about 3/8 inch. The cuts made in the stock must be the same as at the base of the scion, which should have 2 healthy buds. First a long cut, 5/8 to 1.5 inch is made, smooth and oblique. When working with larger material, longer cuts are made. On each of the surfaces a cut in the opposite direction is opened, executed from top to bottom, starting from a point situated about a quarter of the distance from the edge and about half as long as the first cut.

The stock and scion are thus threaded into each other, with the 2 tongues fixed. It is extremely important that the layers of cambium meet at least on one side, but possibly the whole perimeter. The united scion and stock should be tied across their junction, otherwise healing will not occur. Raffia makes a good tie, covered with grafting tar or wax. Otherwise, use a special adhesive tape which stretches with plant growth. (PARAFILM works well-L. G.)

WHIP GRAFT: a) the scion with a long oblique cut to the base; b) the stock and scion about to be joined; c) the two parts tied together; d) covered with tar or wax.

CLEFT GRAFT:
This graft should be completed at the commencement of spring, a few days before the bark graft, when the buds of the stock are beginning to swell, but the sap is not yet running.

Scions, only one when the stock is smaller than 3/4 to 1 inch, have a base cut to shape a wedge and should be placed so that their cambium is in contact with that of the stock. Since the bark of the stock is usually thicker than that of the scion, you should place the scion slightly farther inside the external surface of the stock. Tie the graft securely and cover with plenty of wax or tar.

 Cleft and bark grafts have similar success rates; to improve the cleft, use stocks with diameters less than 5/8 inch and a single scion; or match the diameter of scion and stock.

 CLEFT GRAFT: a) scion with base cut to form a wedge; b) diameter cleft on the stock previously tied an inch below the surface of the cut; c) inserting the scions and tying up the graft; d) covering with grafting tar or wax.

TRIANGLE GRAFT:
This graft can be executed through a 2-3 month period, beginning when sap runs. This method is very difficult for beginners, but expert grafters execute it rapidly and obtain optimum results.  The scion with 2 buds should have the bottom cut in a wedge, with the external side much more broad than the internal ones, to copy the cuts in the grafting stock. The wedge cuts on the scion should be 1 to 2 inches long and smooth. After cuts on the scion and stock have been made, the scion is pressed with force into place. the cambial layers must meet for tight contact. The bark of the scion being finer than that of the stock, it is placed slightly inside the bark of the stock.

Triangle grafts tend to be less subject to breakage caused by wind and snow as compared to bark and cleft grafts. If stock with a diameter about 3/4 inch is used, the graft surface seals immediately, limiting blight attacks and dehydration.

 TRIANGLE GRAFT: a) the scion has been cut into triangle on the base; b) removal from the stock of a triangle of wood; c) insertion of the scion, tied with raffia; d) covering with tar or wax of the surface of the cut & top of the scion.

FLUTE GRAFT (uses scions collected at grafting time):
This graft is largely used in Italy for chestnut trees because it permits good results, also it implies a notable skill on the part of the grafter and a considerable waste of grafting scion.

The flute graft can be executed as soon as the stock and the scion are in sap, (Italy) from mid-April onwards. This consists in removing a ring of bark complete with a bud from the scion. The stock should be cut at the height at which it is intended to graft. Three or 4 vertical cuts in the bark are made, an inch long, and then cut off, or alternatively, a ring of bark identical in dimensions to that of the scion is removed. The scion ring is inserted on the stock and is made to slip until the latter blocks it . With this graft, tar or wax are not used; instead an accurate cut is necessary. The edges of the bark of the stock should cleanly meet the edges of the scion ring. The incidental part of the stock remaining should be cut off with a knife.

 FLUTE GRAFT: a) the stock was cut off; then, 3 to 4 cuts in the bark an inch long are detached; b) inserting of a ring of bark scion containing one healthy bud and taken from the variety to be propagated; c) exposed wood above is cut off.

 The Italians use paper bags to provide shade the first 40 days following grafting. They recommend at least three times following grafting that you return to remove suckers from the root, otherwise install a thick paper mulch around the stock, held down by stones to discourage suckers. Beside each graft, a wood or metal stake about 10 ft tall is driven deeply into the ground. A pad of cardboard or other material is placed between the graft and the stake to avoid scraping the bark. The new growth from the graft is secured to the stake after it reaches a height about 20 inches. The tie is repeated every 20 inches & a final time in fall in order that the weight of snow does not bend or break it. Scions which do not take are eliminated as soon as possible because they impede healing of the wound. Grafts tied to stakes should be loosened in the beginning of July (the next year) to avoid strangling, especially for stocks of small dimensions, and the tie is made with tubular plastic or cord.

 Don't be discouraged by early troubles. Competence comes with experience & results more than repay the effort! 

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