HortResearch Publication - Topworking of Citrus Trees by Bark Grafting

HortResearch Publication - Topworking of Citrus Trees by Bark Grafting

Pauline Mooney¹, Peter Anderson¹ and Ryan Duffy² - ¹HortResearch, Kerikeri Research Centre ²Sorrento Orchard, Waimate North Road, Kerikeri

Introduction

Citrus growers must constantly introduce new cultivars into their orchards, to satisfy changing consumer demands. To achieve this it is often cheaper to topwork existing trees than to remove a block and replant with nursery trees. The advantage of topworking is that trees usually come back into production sooner than nursery trees because they have an established root system.

Topworking of citrus trees can be done by either budding or grafting. Minor variations of both methods may be used on trees of all sizes. However, young trees are usually budded and older trees bark grafted (Platt and Opitz, 1973). General points to consider regarding the options when reworking a citrus block are outlined in Table 1.

We have investigated stick grafting techniques into scaffold branches and the basal rootstock of 4 to 15 yr old trees, at different times throughout the growing season. We have found that topworking with fresh graftwood, into either the scaffold and basal rootstock in late spring and summer (when there is an abundance of sap flow) resulted in the highest percentage take.

The following report gives a breakdown of the procedure of grafting.and the advantages and disadvantages of grafting into the tree scaffold or basal rootstock.

To Topwork	To Replant

1. Physical and monetary cost involved in the removal and disposal of canopy, and topworking of the stumps? What is the best season to topwork, and what percentage of successful take can you expect?	1. Physical and monetary cost involved in the removal and disposal of the entire tree structure, and preparation of the ground for replanting?
2. Does the existing plant spacing suit the growing habit of the new cultivar?	2. How many trees are required to fill the block? How much does each tree cost?
3. How much effort is involved in managing the young grafts - staking, pruning and desuckering?	3. How much effort is involved in desuckering and staking the young tree?
4. What is the availability of graft/bud wood?	4. What is the availability of young budded trees?
5. How long before the new tree can be cropped? What percentage yield can be expected?	5. How long before the topworked tree can be cropped. What percentage yield can be expected?
6. Are the new scion cultivar and interstock compatible? Is the tree to be reworked free from disease?

Table 1: Points to consider when deciding to invest in a new cultivar

Method

The methods of grafting into scaffold branches and basal rootstock are very similar and the following steps may be used in either case.

Removal of 'Top' Growth
If grafting into the scaffold branches, a few lower limbs are left to provide shade for the new graft and photosynthates for the root system. If grafting into the trunk or rootstock, remove all of the canopy. The advantages and disadvantages of grafting into either scaffold branches or the basal rootstock are covered in Table 2.

Preparation and Placement of the Stick Graft
Using fresh graftwood, and a sharp budding knife, make a sloping basal cut about 4 - 6cm long on one side of the graft stick (Figure 1). A second cut of about 2cm long is made on the opposite side so that the end of the stick is tapered. Finally a third cut is made to take a sliver off one side of the tapered base.

Basal Rootstock	Scaffold Branches

ADVANTAGES
No competition from excessive original scion regrowth	Nurse branches are left on the tree to provide shade for the graft and to maintain the root system
The new cultivar is grafted closer to the ground	Should the graft fail, there is sufficient structure onto which a second graft may be made
Removal of interstock interference (if grafted into the original rootstock	If the new scion cultivar does not perform to expectations, the tree may be allowed to revert back to the original scion. The presence of an interstock may be an advantage if new scion and rootstock are incompatible
DISADVANTAGES
Root stress due to removal of the total canopy	Vigorous regrowth of the original scion (which competes with the graft and therefore requires constant suppression)
Susceptibility to herbicide damage	Potential interstock incompatibility which could result in either death of the graft, or detrimental effects on fruit quality
If the graft fails there is no second chance for regrafting	Position of the graft, which due to the original tree shape and size, is usually high in the tree and consequently the resultant new canopy is taller than a buded seedling tree

Table 2: Advantages and disadvantages of bark grafting into the basal rootstock and scaffold branches

A cut about 4 - 6cm long is then made in the stock. Bark on one side of the cut is lifted using the budding knife (if placing more than one graft stick in the stock, then the placement of the sticks should be consistently clockwise or anti-clockwise). The prepared scion is then inserted and pushed down into the stock with the longer cut surface facing the wood of the stock and the shorter cut surface facing the bark (Figure 2). The third cut surface should fit snugly against the unlifted bark of the stump.

Protecting the Graft
Scions are greased at the top of the stick and at the junction between the stock and stick to prevent desiccation. The cut surface of the stock is painted with 'BacsealR' to prevent disease (Figure 3).

Scions are held tightly in place by strong 'Insultape' which can be stretched around the stock, covering the stock/stick graft union. If the stock is irregular, sloping or wet, this tape can slip and damage the grafts . To prevent this, a few nails or staples should be placed at the top of the tape.

A thin film (single layer) of cling wrap is placed around the scion stick to prevent dehydration (Figure 4), then a large paper bag is secured over the grafts and stock. This cover provides a degree of protection and shade for the grafts and new shoots. The new shoots will grow through the cling wrap providing the film is not too thick. Once the shoots have reached about 6cm in length the paper bag can be lifted and the shoots staked if necessary.

Basal cut Inserting scion Protecting surface

Figure 1: Preparing the 4 - 6cm sloping basal cut Figure 2: Inserting the scion into the stock Figure 3: Protecting the cut surface of the stock against disease

Preventing dehydration Callus development Vegetative growth

Figure 4: Cling wrap is placed around the stick to prevent dehydration Figure 5: Callus development Figure 6: Vegetative growth on a January grafted basal rootstock (3 months after topworking)

Callused union Vegetaive growth

Figure 7: Vegetative growth on a December grafted basal rootstock (4 months after topworking) Figure 8: Callused union between scion and stock of a December graft Figure 9: Vegetative growth of a December graft into scaffold branches (4 months after topworking)

Discussion

Preliminary results of the three topworking trials are presented in Table 3. Bark grafting into the basal rootstock resulted in a 83 % success rate using Miyagawa satsuma graftwood and between 90 - 96 % success rate using Richards Special graft wood. Bark grafting into the scaffold branches using Richards Special graftwood resulted in 100 % of topworked trees having one or more successful grafts.

A second trial which involved grafting Richards Special onto grapefruit scaffold branches was a total failure. Although we are not completely sure as to the reason for this it has been suggested that NZ grapefruit strains carry a particularly virulent strain of Tristeza virus, Richards Special is particularly susceptible to this virus and this might account for the failure of this trial.

TREATMENT	No. of trees	No. graft sticks per stump	% trees with 1 or more healthy grafts

TRIAL 1
TRUNK GRAFTING
Miyagawa satsuma mandarin onto P.trifoliata Stump age: 4 yrs	12	3	83

TRIAL 2
TRUNK GRAFTING
Richards Special mandarin onto P.trifoliata Stump age: 15 yrs
Grafted December 1990	10	4	90
Grafted January 1991	52	3	94
Grafted February 1991	29	3	96

TRIAL 3
SCAFFOLD BRANCHES
Richards Special mandarin onto Seminole tangelo Stump age: 8 yrs	13	2	100

Table 3: Results of bark grafting trials either into the basal rootstocks or scaffold branches

Wrapping the graft stick in cling wrap, and covering the grafted limb with a brown paper bag, stopped the graft sticks drying out and kept them cool. Grafts made in January and February grew very slowly with bushy shoots, due to a number of the buds on the graft stick sprouting (Figure 6).

The leaves on these shoots did not fully expand prior to the onset of winter and consequently the shoots were not properly hardened-off. Grafts made in December grew quickly and generally produced a single upright shoot (Figure 7 and Figure 9). This growth hardened off prior to the onset of winter.

Topworking in December also allowed the base of grafts to become well callused over (Figure 8) and consequently stronger than those performed later in the season (Figure 5).

The most important factors determining the success of the techniques appeared to be the use of fresh healthy graft wood, the tight binding of the graft stick to the stump, and protection of the graft from dehydration.

Conclusion

The changing market trends which have been responsible for poor returns from citrus such as NZ grapefruit and Seminole tangelo, reinforce the need to improve on the traditionally grown citrus cultivars. Topworking of citrus is an attractive alternative to replanting as more of the recently imported citrus cultivars are released for commercial propagation.

However, growers must consider each situation carefully with regard to rootstock, interstock and scion compatibility. We suggest that if growers are considering topworking their trees that they practice on a few trees initially, thereby perfecting their technique and ruling out problems such as we encounted with Richards Special on NZ grapefruit interstock.

Further trial work will evaluate the merits of topworking earlier in the growing season (October/November) to maximise the growth of the scion prior to the onset of winter. The effects of interstock on fruit quality, the growth patterns and canopy shape of the new scions in either the scaffold branches or basal rootstocks and the need to tailor management strategies to accommodate these effects will also be studied.

Acknowledgements

We would like to thank Murray and Laurel Strang, Keriland Orchard, and Rick and Nicki Curtis, Curtis Orchards, for allowing us the use of trees on their properties for the purposes of this trial.

References

Platt, RG. and Opitz, KW. 1973. The propagation of citrus. In: The Citrus Industry. III. Production Technology. [Reuther W. ed.]. Univ Cal, Agric Sci. pp 37-40

Source: The Orchardist, August 1991

Copyright © 1997 The Horticulture and Food Research Institute of New Zealand Ltd. All rights reserved. Reproduction in whole or in part in any form or medium without express written permission of The Horticulture and Food Research Institute of New Zealand Ltd is prohibited.


Figure 1: Preparing the 4 - 6cm sloping basal cut	Figure 2: Inserting the scion into the stock	Figure 3: Protecting the cut surface of the stock against disease

Figure 4: Cling wrap is placed around the stick to prevent dehydration	Figure 5: Callus development	Figure 6: Vegetative growth on a January grafted basal rootstock (3 months after topworking)

Figure 7: Vegetative growth on a December grafted basal rootstock (4 months after topworking)	Figure 8: Callused union between scion and stock of a December graft	Figure 9: Vegetative growth of a December graft into scaffold branches (4 months after topworking)