An exposé on pome fruit rootstocksResearch & Extension
The Future Orchards® theme in the next 12 months is looking at ‘future trees’ – this article is the first discussing this new theme and focuses below the ground on rootstocks.
For well over a century and, in the case of Europe many centuries, apples and pears have been grown on clonal rootstocks. In that time, many different rootstocks have been developed and at present there are a number of rootstock breeding programs around the world, including in Australia.
Most of our present rootstocks were either identified by East Malling Research Station, UK, from a collection of nursery rootstocks (the Malling Series) or bred by crossing the Malling Series with Northern Spy. The Malling stocks generally, were very susceptible to woolly apple aphid (WAA), a serious insect pest of apple, including apple roots. While not a big problem in colder climates, WAA can be a serious problem in the mild winter climates found in South Africa, Australasia and South America. With the establishment of commercial apple orchards in the southern hemisphere over a century ago, it was found necessary to plant on WAA resistant rootstocks, namely Northern Spy.
Early last century the Malling series were crossed with Northern Spy to breed a line of WAA resistant rootstocks. These WAA tolerant rootstocks were the Merton (M) rootstocks, such as M778 and M793, and the Malling/Merton (MM) rootstocks including MM106, MM102, MM111 and more recently MM116.
M778 is reported to be used in Queensland and M793 has been widely used in South Africa and New Zealand. MM106 has been the most widely planted MM rootstock in Australia and New Zealand for semi intensive orchards, but because of its high susceptibility to Phytophthora root disease, it is now not widely planted and definitely not recommended for replants. MM106 is being superseded by MM116 which has good Phytophthora tolerance. MM102 is very similar to MM106 but is less uniform in its vigour and consequently not widely used. MM111 is reputed to have better drought tolerance but can be more vigorous and less precocious than other medium vigour rootstocks. It has been widely used in South America and sometimes in Australia, particularly Tasmania.
Now that apple orchard growing systems have moved to higher density planting systems these semi-vigorous rootstocks are largely being replaced by more precocious dwarfing rootstocks. However, there is still a place for them with weak-growing precocious scion varieties or plantings on very low vigour sites.
Two other situations where they have a place is for harvest management, because relative to dwarfing rootstocks, fruit maturity on these rootstocks is delayed by seven to 10 days. The other reason for planting on semi-vigorous rootstocks is to minimise fruit russet incidence for varieties prone to russet.
By adopting a multi-leader tree form along the row, it is possible to reduce their vigour by spreading it along the row into two or more leaders. With this approach, between-row spacing can be held at intensive orchard distances and the number of leaders along the row at similar distances to higher density intensive single leader plantings on dwarfing rootstocks. The trick with this approach is to settle the trees into cropping early and maintain regular cropping because regular cropping is a powerful vigour control agent and necessary to control excess vigour with these rootstocks.
M9 has become the most widely planted apple rootstock in the world. It is very adaptable to growing conditions and can be found from the northern limits of apple growing, well above latitude 55N, right down into the subtropics in places like Mexico and in Brazil and Queensland in the southern hemisphere. It is very adaptable to soil type and contrary to popular belief, quite drought tolerant once established.
There are many different clones of M9 with a range in vigour from a little more than Budagovsky 9 through to just below M26 vigour. NAKBT337 is among the lowest vigour group with Emla 9 in the middle, with Pajam 2 (Cepiland) and Nic 29 at the high vigour end of the range. The M9 tree size range is from 30 to 40 per cent of standard seedling rootstock.
M9 clones are all very similar in their nature with brittle roots that need very good tree support, but they also have good graft or bud unions. Susceptibility to WAA is the main pest and disease problem M9 has. That, along with the lack of virus-free planting material, has been the main reason for slow uptake of planting on M9 rootstocks in Australasia and other southern hemisphere countries. Free of known virus (FKV) planting material is necessary for good M9 tree growth.
M9 clones are reported to have good tolerance to Phytophthora root rots. While considerably more tolerant than MM106, our experience has been that Phytophthora root rots can be found on M9 if it is growing in soil conditions that favour the disease.
M26 has become a major rootstock in Australia for intensive apple orchards. There is nowhere else in the world where it has gained such importance, probably because intensive plantings elsewhere are predominantly on M9 clones. Relative to M26, M9 gives a much more uniform tree size, is about 10 per cent less vigorous than M26 and gives a tree better adapted to intensive orchard systems. M26 can be rather variable in tree size and for this reason orchards on it tend to be less productive than those on M9.
M26 has a weak union between scion and rootstock so needs very good tree support right from planting. It is susceptible to Phytophthora root disease and sensitive to specific apple replant disease (SARD). M26 gives large fruit size and is one of the rootstocks that matures fruit early. It also lacks WAA tolerance.
Budagovsky 9 (Bud9)
Slightly less vigorous than M9, Bud9 shows resistance to collar rots and the rootstock itself is resistant to powdery mildew and scab (black spot). It grows well on most soils but needs good irrigation on light sandy soils. It is best suited to high vigour varieties on stronger soils.
It has been used a little bit in Australia but is gaining popularity in North America because of its lower susceptibility to fireblight than the M9 clones.
Ottawa 3 (O3)
The O3 rootstock was bred in Canada, has a similar tree size to M9 and has been used in Australia for intensive plantings. It is reported to be resistant to Phytophthora collar rots but lacks WAA tolerance and is very sensitive to virus, consequently it is no longer favoured for planting.
The Cornell-Geneva Series (CG)
The North American pome fruit industry has consolidated all of its pome fruit rootstock breeding work at Cornell University’s Geneva Research Station. Consequently, there are a lot of very useful rootstocks starting to be released from this program.
Their main focus has been on developing very precocious, dwarfing and semi-dwarfing rootstocks for intensive orchards. Tree form, as well as pest and disease resistance have been key selection criteria. The key resistances they have been selecting for are Phytophthora (collar rot), WAA, SARD and fireblight.
I note that several of the Cornell-Geneva rootstocks have been brought into Australia and are listed in Gordon Brown’s article ‘New apple rootstocks coming soon to your nursery’ published in Australian Fruitgrower (April 2011).
The CG rootstocks mentioned as becoming available through the Australian Nurserymen’s Fruit Improvement Company (ANFIC) are G16, G41, G202 and G201. G16 and G41 are in the M9 vigour range. G16 is very sensitive to latent viruses so will need a scion with FKV health status. It produces very wide crotch angles. G41 gives higher yield efficiency than M9, is very resistant to Phytophthora and appears to be tolerant of SARD. G16 has no WAA resistance. G11 is also available in Australia and gives a tree of similar vigour to M26.
G202 gives similar or slightly larger tree size than M26, has WAA resistance and Phytophthoraresistance. G202 was listed as available in the ANFIC 2014 catalogue but I have not found any evidence of it being available as finished trees in Australia yet. G202 has been commercially available in New Zealand since around 2005 and it is now the second most widely planted rootstock in new plantings after the M9 group. It is very productive, giving very uniform tree size with wide crotch angels.
Although it has good Phytophthora resistance we have observed it to be rather sensitive to soil waterlogging and poor soil aeration. It therefore, needs well drained soils. Weak unions can be a problem with it so it needs very good tree support. Union strength tends to be linked to scion variety with Fuji appearing to give the strongest union. Royal Gala is not too bad either. Scifresh and particularly Scilate tend to be brittle at the union. For very precocious varieties we are tending to use similar tree densities as would be used for our stronger M9 clones. G202 performs well as a replant too.
There is some confusion around the nomenclature of C210 rootstock. North American descriptions rate it more vigorous than G202 whereas our experience with the one we have called G210 indicates it gives a smaller tree than G202, much more in the M9 size range. The one we have appears to have very good SARD tolerance. It is difficult to propagate so it is not liked by the nursery industry.
The WAA Problem
We should not ignore the lessons from history in regards to this serious orchard insect pest. In recent years, there has been widespread movement towards WAA susceptible rootstocks and Australia now has around 50 per cent or more of its orchard area planted on WAA susceptible rootstocks. Furthermore, the proportion of orchards planted on WAA susceptible rootstocks continues to increase.
WAA root infestation is capable of causing severe tree stress, not to mention the problem of fruit, bud and wood infestation caused by the WAA root population migrating upstairs. High levels of root infestation stunts tree vigour and, in South Africa, WAA root infestation has been linked to Pseudomonas syringae disease injury in buds and fruit.
At present, we have in Australia some very good pesticides, mainly the neonicotinoid group, that are giving excellent WAA root infestation control. However, relying solely on pesticides for WAA control may be a high risk strategy in the long run because:
- Aphids in general can develop resistance to pesticides, so it is probable that resistance management strategies will be necessary to maintain the good control we now have.
- There may not be replacement chemical controls available when eventually resistance to our present insecticides occurs.
- Consumers are becoming more and more sensitive to the use of agricultural chemicals, making it very difficult to introduce new pesticides.
WAA root infestation tends to be related to soil type. Soils which allow easy access to the roots for the aphids, such as soils prone to cracking or coarse sands tend to suffer more WAA root injury than other soils. Now that some good dwarfing rootstocks with WAA tolerance are becoming available, it would be short sighted of the apple industry not to adopt them in place of the WAA susceptible rootstocks that dominate plantings at present.
Calleryana D6 has been the dominant pear rootstock used in Australia for many years. As with apples, there is a move towards lower vigour pear rootstocks planted more intensively. Generally, around the world, various Quinces have been used for intensive pear plantings. With the exception of Doyenne du Comice and Beurre Hardy, most European pear varieties are incompatible with Quince so need to use one of these varieties as an interstem.
In addition to Quince rootstocks, there are two semi-vigorous pear rootstocks available in Australia. These are the South African BP1, which is clonally propagated but rather difficult to root, and the locally developed BM2000. In order of descending vigour:
- Calleryana D6.
- BP1 – 75 per cent of D6 tree size, needs well drained soils.
- BM2000 – smaller tree than D6 and more precocious coming into crop several years earlier than D6. 70% of D6 tree size.
- Quince BA 29C – a provence quince, higher vigour than the other quince rootstocks. 50-65 per cent of D6 tree size.
- Quince A – 50 per cent of D6 tree size.
- Quince C – about 40 per cent of D6 tree size, weakest growing and most precocious of the quince rootstocks. Very good rootstock for shy bearing pear varieties.
Pears on quince rootstocks, particularly Quince A and C need very good tree support. Quince rootstocks are intolerant of alkaline soils and can suffer severe iron induced chlorosis when planted on these soils.
This project has been funded by Horticulture Innovation Australia Limited using the research and development apple and pear levy and funds from the Australian Government.