For many years, seedling apple rootstocks were commonly used for grafted trees. In North America, seeds of commercial cultivars, such as Delicious were obtained from processing or juice plants and grown for rootstocks. Seeds of French Crab and Antonovka were also imported for seedling rootstock production. However, dissatisfaction with variability in tree size, intolerance of climatic or soil extremes, and lack of pest resistance of seedling rootstocks led to the development of clonal rootstocks at the East Malling Research Station, Kent, England beginning in the early 1900's.
Today the most common clonal rootstocks from the East Malling program are the Malling (M.) or Malling Merton (MM) series, including M.9, M.26, M.7, MM.106, and MM. 111. Other clonal apple rootstocks include the Budagovsky (Bud), Polish (P.) Vineland (V.), Supporter (S.) and the Geneva series (Table 1). The Cornell Geneva breeding program was initiated in 1968 to develop rootstocks with improved disease resistance (especially fire blight) and tolerance to low winter temperatures.
One of the main reasons clonal rootstocks are used is to control tree size or vigor. Interstems, which are three-part trees with an additional rootstock piece grafted between the scion cultivar and the lower rootstock, are also used to reduce tree size. With small trees, placed at close spacings, a greater number of trees can be plant per unit area of land. Thus, high density planting is a strategy used to increase apple production. Small trees are also desirable for homeowners with limited space for trees in full sun. Table 1 lists the relative vigor of rootstocks currently used by commercial nurseries. However, many other clonal rootstocks have been tested at the University of Missouri for more than 30 years.
Another important attribute of clonal rootstocks is their capacity to induce precocity or apple production at a young tree age. For example, dwarf trees on a M.9 rootstock generally bear fruit two to three years after planting. In contrast, a less dwarfing rootstock, such as MM.11 generally does not bear fruit until age five. In many cases, the more dwarfing the rootstock, the sooner fruiting is initiated.
Rootstocks also influence tree anchorage and root brittleness. Generally, more dwarfing rootstocks have shallower root systems and more vigorous rootstocks are deeper rooted. However, depth of rooting is also influenced by soil type, compaction and drainage. Some of the dwarfing rootstocks, such as M.9 and B.9 have short wood fibers which makes them brittle. For this reason, subclones of M.9 rootstock, such as M.9 Nic 29 have been selected with less brittle roots. Under windy conditions (>40 mph) large structural roots break cleanly at the base of the trunk. Trees on these rootstocks, as well as those as vigorous M.7 should be supported by a stake or trellis structure to prevent leaning or tree loss.
Clonal rootstocks also vary in their adaptability to soil textures, temperatures, and moisture content. For example trees on MM.111 grown in heavy clay soils generally perform better than those on M.9 rootstock. Also, MM.111 and M.7 trees are relatively resistant to high soil temperatures, whereas M.9 trees perform poorly at soil temperatures >77°F. MM.111 also survives summer drought conditions better than many other rootstocks. However, MM.111 has poor survival in flooded soil conditions during the growing season as compared to M.7, M.26, and M.9 rootstocks.
Rootstocks also vary in their capacity to cease growth in the fall, acclimate to cold winter temperatures, and to initiate growth in the spring. M.7 rootstock induces early vegetative maturity in the fall, resulting in resistance to early winter resistance to freezing. However, M.7 and M.9 rootstocks have poor mid-winter hardiness as compared to B.9, P.2, P.22, Antonovka 313, and most Geneva rootstocks.
Pest resistance of apple trees is also influenced by the rootstock (Table 2). The MM series were developed specifically to resist woolly apple aphids which infested many of the orchards in England, Australia, and New Zealand. Infestations of dogwood borer are often less severe on rootstocks that rarely produce burrknots (root initials on the trunk), including B.9, G.41, G.16, and G.65. In contrast, M.9, M.26, M.7 and MM.111 rootstocks frequently develop burrknots and are more susceptible to dogwood borer injury. Pine and meadow vole feeding is particularly severe on rootstocks with thick bark, such as M.9.
Two predominant diseases affecting apple rootstocks are fire blight and crown rot. Fire blight can cause rapid death to trees on M.9 or M.26 rootstocks or in trees with these used as an interstem. G.16 and G.41 are two of the more fire blight rootstocks in the Geneva series. Fire blight infection can also occur on root suckers or burrknot and can be translocated to the trunk, resulting in tree death. M.9 and M.26 also prolong the bloom period and cause flowering on one-year old wood. This longer bloom period increases the likelihood of optimum weather conditions that result in fire blight infection. Also, the combination of an extremely fire blight susceptible cultivar (Jonathan, Gala) on a susceptible rootstock should be avoided. On sites with poorly drained soils, rootstocks such as B.9 or G.16 are recommended for their resistance to crown rot, whereas MM.106 and M.7 should be avoided.
A relatively new virus in the U.S. is apple union necrosis, which is caused by tomato ringspot virus and is spread through the soil by dagger nematodes. Affected trees break cleanly at the graft union. Trees on MM. 106 and M.26 are susceptible, especially with Delicious as the scion cultivar. In contrast, M.7 rootstocks in combination with less sensitive cultivars (Empire, Golden Delicious, York) appear to be resistant. In the original Malling rootstocks, latent viruses are also present, but have no apparent adverse effects on apple trees. In an attempt to remove viruses, but not all latent ones, some (rubbery wood, apple mosaic, star crack, and chat fruit) were removed and these rootstocks were designated with an "A" (e.g., M.7A, M.9A). Later efforts to produce virus-free rootstocks resulted in the EMLA designation (M.9 EMLA). This effort resulted in a slight increase in tree size with most rootstocks. However, the greatest increase in size occurred with M.9 EMLA, which can be as much as 50% larger than the original clone. In conclusion, clonal rootstocks provide many beneficial attributes to apple production, but some that are available in the marketplace also have shortcomings. Thus, whenever possible, carefully select a rootstock that best matches your site conditions to avoid many of the problems that plague apple growers.
|Table 1. Influence of Commercially Available Apple Rootstocks on Tree Vigor|
|M.9 Nic 29||X|
|M.9 NAKB T337||X|
|M.9 Pajam 2||X|
|Table 2. Susceptibility of Selected Apple Rootstocks to Various Disease|
|Rootstock||Crown Rot||Fire Blight||Latent Viruses|
|Rating System: S=susceptible, MS=moderately susciptible, M=intermediate, MR=moderately resistant, R=resistant, VR=very resistant, T=tolerant, and NT=not tested.|
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REVISED: December 6, 2011