Topworking of outdated citrus cultivars is an alternative to replanting an orchard block. However, topworking of citrus as late as January results in the young tender shoots being frost damaged during winter. The aim of this research was to identify the best time for topworking under New Zealand conditions and identify aids for increasing grafting efficiency.

Introduction

Whenever a cultivar of superior quality is released or when demand for the existing cultivar is weakening, topworking by patch or shield budding into the scaffold branches is carried out (Figure 1). Within a single orchard block you will commonly find the remains of an old rootstock (generation 1) and the current cultivar in production (generation 2). This tree may have a portion of the canopy removed, the exposed scaffold branches of which will be budded with the latest 'in vogue' cultivar (generation 3). Alongside these trees will be planted a rootstock seedling (Figure 2) which will be budded once the topworked cultivar is surpassed by another improved selection.

Figure 1: Topworking by patch or shield budding into the scaffold branches is the preferred method in Spain	Figure 2: In Spain it is common practice to plant a rootstock seedling alongside each mature citrus tree, this rootstock will be budded once the present cultivar is surpassed by another improved selection

Our first article on topworking^[1]suggested that topworking of uneconomical citrus cultivars is a viable alternative to replanting in New Zealand. Topworking success and later growth of the scion, provided that the method used is correct, will depend largely on the time of year grafting is carried out. The graft requires enough time for callus to form and for the shoots to harden off before winter. Good callusing between the scion and stock provides stability for the growing scion. A poor union may result in many of the scions being blown off the stocks. In this article we look at the effects that timing , pruning pastes and anti-transpirants have on the success rate of topworking.

Only trees of good size and health should be considered for topworking. Trees which show disease symptoms or old trees with declining vigour are not suitable for topworking. The use of herbicides such as diuron and bromacil should not be used on trees for at least a year prior to topworking^[2], as these herbicides are harmful to the developing shoots.

When choosing graftwood for topworking, follow the same guidelines used when selecting budwood for propagating nursery trees. Use the best available, healthy true-to-type wood off superior clonal selections. Ideally these trees would have been virus-indexed, but in the short term such material is not available in New Zealand. Graft transmission of diseases caused by viruses and viroids is of major concern^[2].

Eventually, viroid free budwood material will be available to growers from the New Zealand Citrus Budwood Scheme, which was established at Kerikeri Research Centre earlier this year. In the meanwhile, take graftwood only from healthy trees older than 10 years which show normal stocks.

Grapefruit cultivars are sensitive to citrus tristeza virus (CTV) and therefore topworking into grapefruit interstocks may result in a poor success rate^[1],[2]. Unsatisfactory results have also been recorded in Australia when topworking lemon trees, due in this instance to the lemon trees being infected with shellbark and other pathogens, which in time may severely affect the vigour and productivity of the new scion^[2].

We recommend when topworking lemon and grapefruit trees that the grafts sticks are placed directly into the trifoliate rootstock. If this is not possible then the block should be replanted rather than topworked. The same rule holds true for trees which have marked signs of tristeza stem pitting.

Time of Topworking

The best time for topworking was during December, when 90% of grafts took, and 100% of the topworking trees produced a healthy, vigorous new scion. The percentages of successful grafts on trees topworked during October, November, January and February were significantly lower than on trees worked during December. The topworking success during October was very poor, due to poor sap flow in the stocks. The vigour of scions from the January/February period was not as good as the November/December grafting. This was due to climatic conditions becoming too cool for good callusing and shoot growth before the onset of winter.

Aids for increasing grafting efficiency

A number of anti-transpirants are available on the market which are reported to reduce moisture loss by inhibiting evaporative transpiration. It was suggested that this might be a cheap and more effective alternative to cling wrap. Therefore an anti-transpirant planting spray (marketed under the trade name Lac Balsam® Planting Spray) was included in the trial. The planting spray is marketed for the prevention of water loss from plants following transplanting. It is a milky white solution which dries quickly, leaving a nearly invisible white film on the plant material. This coating is not toxic to plants and has the advantage that it does not restrict bud break and shoot extension. Lac Balsam Planting Spray is sold in concentrated form and was used at the recommended dilution rate of 1:4 v/v (concentrate:water).

The stump of the grafted tree also requires treatment to prevent moisture loss and infection by air and soil borne pathogens. Two pastes were trialed, Bacseal® (Bayer NZ Ltd) and Lac Balsam® Artificial Bark (Ciba Geigy NZ Ltd). Bacseal is a water repellent fungicide and bactericide dressing, marketed for sealing pruning wounds in fruit and ornamental trees. The Lac Balsam Artificial Bark is an artificial resin which forms a rubbery, elastic covering over the wound site. It is reported to promote callus formation along the edge of the cut. The resin does not include either fungicides or bactericides, but is reported to form a physical barrier over the wound site thereby reducing infection by pathogens and parasites.

The trial was laid down in a randomised block design, with 14 replications. Each plot consisted of single tree. The two scion treatments were applied in combination with each of the two stump treatments. Results as presented in Table 2 were disappointing.

Table 2: Percentage succesful grafts and growth measurements for December grafting using the two scion and stump treatments

The percentages of successful grafts when using the planting spray were significantly less than when using cling wrap. However, the cost effectiveness of using the spray over the cling wrap warrants further investigation into the use of anti-transpirants. There was not a significant difference between percentage graft success using either Bacseal or Artificial Bark as a stock sealant. There were no significant treatment differences in scion diameter.

Recommendations

This results in a high percentage of successful grafts, and allows the young scions sufficient time to harden off before the onset of winter. The preferred method for topworking is the placement of a stick graft into the rootstock of the reworked tree. The scion stick should be protected from drying out by wrapping it in cling wrap. The cut surface of the stump should be sealed using a wound sealant, preferably one which contains fungicide and bactericides in its formulation.

We suggest that between 2 - 3 stick grafts are placed onto each stump, as an insurance that one of the grafts would be successful. It is important, however, that during the following season only one of the successful grafts (Figure 3) is selected and the remainder removed to prevent overcrowding.
  Figure 3: Topworked tree, 18 months after grafting

This also avoids the problem of a bowl developing within the centre of the tree, which might result in hygiene problems at a later date. The scions must be staked for support against wind and mechanical damage.

Acknowledgments

References

1. Anderson P., Duffy R. and Mooney PA. 1991. Topworking of citrus by bark grafting. The Orchardist of New Zealand, 64(7): 37-39.

2. Bevington K., Sanderson G., Barkley P., Gallasch P. and Edwards M. 1991. Topworking of citrus. Australian Citrus News, May: 8-11.

See also:
HortResearch Publication - Topworking of Citrus Trees by Bark Grafting

Source: The Orchardist, October 1992