Intensive planting systems depend on rootstocks which confer significant tree size reduction and induce cropping precocity (high early yields). By growing smaller sized but highly productive trees, intensive planting systems are possible using planting densities two to four times higher than in intermediate vigour planting systems. Internationally the most widely used dwarfing rootstock is Malling 9 (M.9) with Malling 26 (M.26) the most widely planted semi-dwarf rootstock. Quince A, C and 193/16 rootstocks are used for European pear dwarfing rootstocks.

Vigour

These rootstocks are referred to as dwarfing or semi-dwarfing rootstocks because apple and pear trees grown on them typically grow only to 30 to 50% of the size of trees on rootstocks like MM.106 and M.793. Research and commercial experience confirms similar differences in vigour between dwarf, semi-dwarf and intermediate vigour rootstocks occur under NZ conditions. However absolute size of trees can be larger on dwarf and semi-dwarf rootstocks on high vigour sites in NZ than often quoted from the Northern Hemisphere. The 50+% difference in tree size between M.9 and M.26 in NZ, may also be greater than in some Northern Hemisphere growing regions. However alternative methods of tree management also influence ultimate tree size.

Malling 9 dwarf rootstocks can be used as an interstock grafted between an intermediate vigour rootstock and the cultivar. These `interstem trees' usually grow to semi-dwarf size similar to M.26. Our NZ research is finding that interstem tree size can vary because of interstem length and among regional sites. Interstock length of 10-15 cm results in larger trees than interstock length of 20-30 cm. In Nelson some interstem trees have been closer to M.9 in size than M.26 and smaller than similar trees from Hawkes Bay and Otago. NZ studies so far have not found any superior performance from using other dwarf rootstock clones (eg. Mark, Budagovsky.9) over M.9 as interstocks.

Commercially available rootstocks for intensive systems

Apple dwarf rootstocks: M.9, Mark, budagovsky 9
Apple semi-dwarf rootstocks: M.26, M.9 interstem
Pear dwarf rootstocks: Quince A, Quince C, QR 193/16

M.9 sub-clones

Selection for nursery performance in Europe has resulted in a large number of sub-clones of M.9. Some are commercially available in NZ. Comparisons among clones in Europe have found only small differences in tree size and young tree precocity and no differences in overall productivity. It is important to remember that these are all M.9 dwarf rootstocks and differences in vigour between sub-clones are much less than differences between M.9 and semi-dwarfing rootstocks like M.26. HortResearch is conducting evaluations to compare vigour and productivity among commercially available M.9 sub-clones in NZ.
M.9 sub-clones produced in New Zealand:
NZ.9 (DSIR heat treated, free of specified viruses M.9 NZ); EMLA.9 (East Malling Res Station UK); T.337 (NAKB Holland); 719 (Burgmer nursery Germany); Lancep (Pajam 1) (INRA France); Cepiland (Pajam 2) (INRA France); Nic 29 (Nicolai nursery Belgium).

Disease and pest resistance

Rootstock resistance to diseases and pests are important for tree adaptation and fruit quality. Commercial apple rootstocks for intensive planting systems (M.9, M.26 and Mark) have several key pest and disease susceptibility weaknesses (Table 1). All lack resistance to Woolly Apple Aphid (WAA) and are highly susceptible to Fireblight. But these same rootstocks have high resistance to phytophthora root rots. Hence the combination of risks are somewhat different from MM.106 and M.793.

CG 202, CG 210 and AR.86-1-25 are promising new rootstocks in early commercial development which have enhanced resistance attributes. AR.86-1-25 is an intermediate vigour, phytophthora resistant alternative to MM.106/M.793 for use in interstem trees and on very low vigour sites. CG 202 and CG 210 could have very wide application for intensive planting systems because of their semi-dwarfing vigour and resistance for WAA, fireblight and phytophthora (Table 1).

Table 1. Dwarf, semi-dwarf and intermediate vigour rootstocks commercially available or soon-to-be commercially available in New Zealand, relevant to apple intensive planting systems.

Rootstocks and replant situations

Increasingly new apple plantings will be intensive systems on dwarf rootstocks. This means that selection of rootstock will include tolerance to replant conditions and specific apple replant disorder (SARD), as older orchards are replaced. Recognition of replant issues was introduced into HortResearch rootstock research from 1993. We have found a trend for less growth inhibition by replant conditions among semi-dwarf rootstocks compared with intermediate vigour rootstocks. Pre-planting soil fumigation of replant soils also produced a significant stimulation of tree growth on M.9, M.26 and Mark rootstocks. Overseas and commercial experience also indicates that dwarf rootstocks such as M.9 and Mark produce satisfactory tree growth on replant sites. In spite of rootstock tolerance to replant conditions, SARD is highly variable between individual fields so growth inhibition can be equally variable. A pre-planting SARD bioassay should be carried out to establish the degree of growth reduction likely. This information can then be used in calculations of tree density for the particular chosen rootstock to establish the intensive planting system.

What we don't know yet and are finding out?

1. Compatibility of cultivars on new rootstock selections (CG 202 and CG 210)
2. Virus sensitivity of CG 202 and CG 210
3. Mature tree behaviour and performance on new rootstocks and interstems
4. More extensive evaluations of growing situations using grower plantings
5. Introducing new resistant rootstocks into performance evaluations
6. Pre-selecting rootstocks for WAA resistance prior to testing