EARLY RESULTS WITH DWARFING ROOTSTOCKS USED AS INTERSTOCKS FOR APPLES IN NEW ZEALAND

Although systems on semi-vigorous rootstocks such as MM.106 and M.793 have served the New Zealand apple industry well, there is now a requirement for a smaller tree size. All commercially available dwarfing rootstocks are, however, susceptible to woolly apple aphid. Unfortunately New Zealand has a history of root infestation of woolly apple aphid and this was one reason why the Merton and Malling Merton series of rootstocks were developed. Dwarfing rootstocks used as interstocks offer the New Zealand grower the combination of a woolly apple aphid resistant understock with a smaller tree size. Unlike some practices in the USA, it is imperative in this situation that the interstock stempiece is maintained clear of the ground to prevent rooting. Any woolly apple aphids present will then be visible and susceptible to predation from the parasite Aphelinus mali.

Two interstem trials have been planted in several locations within the fruit growing regions of New Zealand. The first trial with cv ‘Braeburn’ was planted in the spring of 1993 and compared variable interstem lengths, from 10–30 cm, of virus-free M.9 (NZ9), with MM.106 and M.793 as understocks and additional trees worked directly on M.9, MM.106 and M.793 as controls. After three growing seasons, the interstock trees at Riwaka and Appleby (South Island) were smaller or comparable in size (TCA and canopy volume) to trees worked directly on M.9, whereas in Havelock North (North Island) the interstem trees were intermediate in size between the trees worked directly on M.9 and directly on the understocks. Length of interstem, at this stage, has a relatively small but consistent effect on tree size. The trees were allowed to crop in their third leaf, and yield per unit TCA for the interstems was intermediate between trees worked directly on M.9 and directly on the understocks at two sites but comparable with trees on M.9 at Riwaka.

The second trial, planted in the spring of 1994, compared a range of dwarfing rootstocks as interstems on MM.106 and ‘Northern Spy’ understocks and ‘Royal Gala’ as the scion cultivar. The interstems were 30 cm of B.491, B.9, M.9, ‘Mark’ and MAC.46 and were selected on the basis of dwarfing, availability and freedom from burr knots and virus. Trees worked directly on M.26 and M.9 served as controls. By the end of the second growing season, trees with an interstem of B.491 were the smallest trees in the trial, while trees with an interstem of MAC.46 were significantly larger than trees on M.26. Yield per unit TCA in the second leaf was lowest on trees with an interstem of MAC.46 and highest on trees directly on M.9 or trees with an interstem of B.491.

Both trials are still young and will be monitored in the future, particularly looking at yield, fruit quality and maturity, and suckering. At this stage there is no interaction between understock and interstock on either trial.