Is There A Future For Interstem Apple Trees In New Zealand?
Traditionally an interstem has been used in New Zealand to overcome problems of compatibility e.g. the use of 'Beurre Hardy' as an interstock for 'Beurre Bosc' on quince rootstock. Recently, however, there has been an increasing interest in using interstem apple trees. In this case the need is not to overcome incompatibility but to combine desirable characters from both the interstem and the rootstock. Although trees on MM.106 and M.793 rootstocks have served the industry well, many growers are now looking for smaller trees, with their advantages of easier management and improved fruit quality. At present, however, there are no dwarfing apple rootstocks that are resistant to woolly apple aphid (WAA). Interstem trees can combine the WAA resistance of a rootstock, such as MM.106, with the size controlling ability of an interstem of M.9. In the long term, the problem will be solved by breeding and this is an objective in the breeding programme at Geneva, USA and in the breeding programme that has recently been started by HortResearch here in New Zealand. In the meantime interstem trees may offer the only sure way of avoiding serious debilitation from WAA.
Woolly apple aphid can flourish on the shoots and roots of susceptible apple rootstocks and cultivars. Rootstocks such as MM.106 and M.793 were bred for resistance to WAA and for trees on these rootstocks, WAA remains principally on the scion cultivar where it is susceptible to chemical sprays and the parasite Aphelinus mali. Unfortunately the parasite only attacks the aphid on the aerial part of the tree, so a tree worked directly on a susceptible rootstock such as M.9 has a secure home for WAA on the root system. It may be that in certain areas of New Zealand and on some soil types WAA will not migrate down to the root system, and in these areas, trees on susceptible stocks such as M.9, 'Mark' and M.26 will not succumb to infestations of WAA on the roots. In other areas, where WAA will readily affect the root system, an interstem tree offers a solution. It is important to remember that the understock part of the interstem tree does not confer resistance to WAA to the interstem section, but it does mean that the susceptible tissue is now above ground where any WAA can be attacked by the parasite.
Early work on interstem apple trees at East Malling (Parry and Rogers, 1972) and in the US (Carlson and Oh, 1975) showed that the length of the dwarfing interstem controlled the tree size. Hence a tree with a short interstem piece was larger than a tree with a longer interstem. Collaborative Dutch and Italian work found that interstem trees on MM.106 with a 35 cm interstem piece of M.9 were comparable in size to trees worked directly on M.9 (Wertheim, Morini and Loreti, 1989). Thus not only can the tree size be controlled by the length of the interstem piece but care must be taken in the nursery to ensure that this length is kept constant, otherwise variable tree size will be the consequence in the orchard.
Traditionally interstem trees have taken three years to produce in the nursery. In the first year the understock is lined out and budded in the summer with the interstem. In the second year, the understock is cut down to the interstem bud which grows up during the summer to be rebudded with the scion cultivar. In the third year, the tree is cut down to the scion bud which then grows out to produce the final saleable tree. Many nurserymen, however, like to rent fresh land for a two year period so interstem trees do not fit in very well to this cycle.
Recent research by HortResearch staff has shown, however, that good quality interstem trees can be produced within two years. A bench graft, using a two-bud graft of the dwarfing interstem, was made onto the understock during early August. These were then left in moist sphagnum moss in cool conditions until planting out in late September. The interstem buds grew away quickly and the resultant shoots were thick enough to bud later that same summer at the normal budding time, at 30 cm above the initial union. Thereafter the treatment of the trees was similar to any normal nursery fruit tree.
Interstem trees were needed for an experiment and the opportunity was taken to compare the tree quality of interstem trees on either MM.106 or 'Northern Spy' understocks, produced by the techniques just described, with trees produced normally on either M.26 or M.9. All treatments were randomised within the same nursery row and all trees, whether rootstocks or understocks with interstem grafts, were planted the same day and received all subsequent treatments at the same time. The measurements of tree quality just prior to lifting in June 1994 are summarised on Table 1. Trunk diameter was taken 20 cm above the highest union and tree height measured from the ground. All feathers were removed below a height of 50 cm. There were no significant differences in tree height or girth between the interstem trees and trees produced directly on either M.26 or M.9. Although the tree height of the interstem trees was taller than that of the normal trees, it must be remembered that the height of the top union of the interstem tree is 30 cm above that of the normal tree.
| Treatment | Trunk diameter (mm) | Tree height (m) | Number of feathers |
|---|---|---|---|
| M.9 interstem on MM.106 understock | 18.8 | 1.86 | 6.9 |
| M.9 interstem on 'Northern Spy' understock | 18.0 | 1.96 | 9.6 |
| M.9 rootstock | 17.1 | 1.65 | 7.3 |
| M.26 rootstock | 17.9 | 1.72 | 8.4 |
| SED | 0.71 | 0.05 | 1.5 |
There are other ways of producing interstem trees within two years. For example, the two grafts for an interstem tree can be made at the same time. In another trial we produced some large trees of 'Beurre Bosc' on BA29 quince with a range of interstems by using two 2-bud grafts.
A few years ago at East Malling, some M.9 interstem trees were raised in a slightly different way. When budding a line of M.9 stocks in the summer, another scion bud was introduced higher up the rootstock. This upper section of M.9, with its scion bud already well callused, was removed and bench grafted to a MM.106 understock. This had the advantage of using the original M.9 rootstock twice, once for the interstem and once for the normal tree. It did mean, however, that the interstem piece was rather thick and consequently large calliper MM.106 rootstocks were needed as the understocks.
Although an interstem tree does offer the advantage of being able to control tree size and have a root system that is resistant to WAA, there may be disadvantages and growers should be aware of them. The introduction of the interstock can lead to some unexpected interactions between the three sets of genes, so that cultivars that would normally be the same size worked on one rootstock can be different in size when worked on an interstem. In some cases, the understock of interstem trees has produced more root suckers than normal (Parry and Rogers, 1968). Sometimes a scion on an interstem tree has not been as precocious as a scion worked directly on a dwarfing rootstock (Barritt, personal communication). As there is little experience of interstem apple trees in New Zealand and because of some of the conflicting experience overseas, HortResearch established plots of 'Braeburn' on either M.793 or MM.106 understocks with variable lengths of M.9 interstem, at Riwaka, Appleby and Havelock North in 1993. Further plots of 'Royal Gala' were established in the spring of 1994 using a range of dwarfing rootstocks as interstems - B491, B9, M.9, 'Mark' and Mac46 - with either MM.106 or 'Northern Spy' as understocks. M.26 has not been included as an interstem as it is prone to burr knots, which could increase the tree variability. Recent Japanese work has found that M.26 makes a poor interstem for this very reason (Koike, Makita and Tsukahara, 1993).
Evaluation of interstem trees is therefore well underway in these trials located in both Hawkes Bay and Nelson. Although there are some disadvantages with interstem trees, they will offer an option for combining vigour control with WAA resistance under New Zealand conditions. The main disadvantage of increased production time has been overcome by a range of methods that allow the more complex tree to be produced in the same time as a conventional scion-rootstock combination.
References
Carlson, R.F. and Oh, S.D. (1975). Influence of interstem lengths of M.8 clone Malus sylvestris Mill. on growth, precocity, yield and spacing of two apple cultivars. J Amer Soc Hort Sci 100:450-452.
Koike, H., Makita, H. and Tsukahara, K. (1993). Effect of an apple chlorotic leaf spot virus free M.9 rootstock on the growth of apple trees. J Japan Soc Hort Sci 62:499-504.
Parry, M.S. and Rogers, W.S. (1968). Dwarfing interstocks; their effect on the field performance and anchorage of apple trees. J Hort Sci 43:133-146.
Parry, M.S. and Rogers, W.S. (1972). Effects of interstock length and vigour on the field performance of Cox's Orange Pippin apples. J Hort Sci 47:97-105.
Wertheim, S.J., Morini, S. and Loreti, F. (1989). Effect of M.27 and M.9 used as rootstock and as interstem on apple tree behaviour in two different growing conditions. Acta Hort 243:37-44.