It is well known that rootstocks are used for tree size control but we may need to remind ourselves of their other benefits. They have other specific influences such as winter hardiness, early yield, good fruit size, phytophora and collar rot resistance, replant disease tolerance and mildew and woolly aphid resistance. The one thing they all have in common is that they produce a uniform stand of trees.
East Malling Research and the John Innes Institute Research station in Kent led the world in rootstock breeding beginning in 1922. Releases started in the '30s (MI series), followed by the '50s (MM series) and the '60s (M series). Sadly this time scale is not acceptable to government funded research these days and rootstock breeding has ceased in this country. Even the ‘new’ M116 apple rootstock arriving on the scene in the next few years was actually bred in the 1960s. However, rootstock breeding continues all over the world and the single purpose is to improve fruit yield, quality and disease resistance, the latter becoming so important with the reducing use of chemicals.
The exchange of influences between rootstock and scion are quite complex and in some cases still unexplained. Growth hormone, auxin, cytokinin and gibberelin exchanges between the rootstock and scion all play their part in influencing vigour, flower inducement and other benefits. Although interesting from a scientific angle we will not dwell on this subject in this article.
All rootstocks themselves are produced by vegetative means with apples and pears from stoolbeds or layers and stone fruit rootstocks from hardwood cuttings or layers. Latterly some of the more recent introductions are produced by invitro techniques (tissue culture) to hasten production in the early stages. The Gisela series of new cherries and various new French plum rootstocks such as Plumina® and Jaspi® are produced this way, in fact they are more difficult to produce by conventional means. M116 has also proved difficult to root on stoolbeds and is being propagated by tissue culture.
There are some very exciting types arriving on the scene from various parts of the world. Most of these achieve exceptional yields combined with equally exciting varieties of good disease resistance and quality. A good example of this is the previously mentioned Gisela series of rootstocks for cherries. Not only is there good size control but also very early production of viable flower that sets fruit even in the second year. This is impressive if compared with Colt where it was quite normal to have to wait for 7 or 8 years for a reasonable crop.
There are some benefits of growing trees on their own roots and this has a keen following in some circles. There appear to be marginal improvements in flavour, keeping quality and disease resistance. This is very subjective and worthy of more study. However, many of the benefits of rootstocks would not be realised with own root trees. The most difficult problem is tree growth control. There are a few naturally low vigour, heavily spurred varieties that perform well on their own roots (Greensleeves, Spur Golden Delicious, Falstaff to name a few) but so many that would produce timber rather than fruit! Triploids would be the most difficult of all which if unattended would delay cropping for many years. Propagation is also more difficult and there would be no benefit of the rootstock influence of disease and pest resistance; an example being woolly aphid in the MM series.
We have been asked to comment on recent observations on the stability of M25 and its ability to anchor well enough in standard orchards. Having spoken to others who have experience in the supply of trees and have gathered information on this subject it appears that these reports are isolated but do raise some interesting observations. On sites where this has occurred the trees affected are generally planted as a full standard tree and have started to crop very early in life. One of the benefits of M25 is its ability to produce fruit from an early age. This often demands too much of the young developing root system in the first few years causing ‘wind rocking’. Fruit thinning and/or heavy pruning may be necessary, please read on.
When any tree is lifted from the nursery (especially 2 years or older) most of its roots are left behind and it is important to balance this by pruning the tree hard before the onset of growth otherwise the head of the tree is disproportionate to the limited root system that can support it in the first year. It is always worth remembering that choice of site and soil is important. Sometimes in our eagerness to plant an orchard (commercial or traditional) we often ignore the need to use the very best soils and aspect.