Rootstock Choices for Ontario Apple Growers

When establishing new apple orchards, producers must not only consider the cultivar to grow when purchasing trees, but also the rootstock onto which the scion has been grafted. The decision is important as it has economic and orchard management consequences, and there are currently a great number of rootstocks available. However, many are not suitable for our climate, cultivars, or orchard systems.

The Tree Fruit Research program at the University of Guelph, Vineland and Simcoe campuses has been actively involved in rootstock research for apple, peach, nectarine, plum, and cherry production in Ontario. This long term research is to assess size controlling characteristics of a number of rootstocks from around the world. The information gathered from these experiments is extremely useful in determining the environment-genetic interactions rootstocks have on yield, precocity, tree performance, longevity, resistance to disease and winter injury, and tendency to sucker. This article will provide an update on some of our most recent results from studies conducted in Simcoe on apples.

Experiment 1 : Performance of Northern Spy, Jonagold and Empire on Commercial and New (Vineland Series) Size Controlling Apple Rootstocks

Eight trees each of Northern Spy, Jonagold (DeCoster), and Empire on eight rootstocks (V.1, V.2, V.3, V.4, V.7, M.26, M.9 T337, O.3) were planted in a silt-loam soil in 1997 at the Horticultural Experiment Station, Simcoe, Ontario, at a spacing of 3.0 m within and 4.5 m between rows (741 trees/hectare;300 trees/acre). Trees were trained to a slender spindle with a 2.5 m support post and were trickle irrigated. Results after nine years of production indicate the following (Figure 1):

• Two rootstocks (O.3 and V.3) were significantly smaller, and two rootstocks (V.4, and V.7) were
  significantly larger than M.26 EMLA.
• Tree size (trunk cross-sectional area) ranged from 74 to 205% relative to M.26E with V.3 being
  the smallest and V.4 the largest. V.1, V.2, and M.9 T337 were statistical similar in size to
  M.26EMLA.
• Significant interactions in performance (yield and tree size) exist between the cultivars and
  rootstocks
• Trees of Northern Spy have had the lowest cumulative yield and largest tree size; Jonagold has
  been the most productive cultivar while Empire the weakest in vigour
• Cumulative yields after 9 years of fruiting indicated that V.7, V.1, and V.3 ranked as the most
  productive. Cumulative yields of M.9 T337 and O.3 have been the least of those tested.
• Suckering has been significantly greater for V.4, particularly for the Northern Spy and Empire
  scion cultivars
• V.3 has been the most yield efficient rootstock followed by M.9 T337, V.2 and V.1
  There was no significant rootstocks effect on mean fruit size

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Experiment 2 : Performance of Honeycrisp and Royal Gala on the Commercial and New (Vineland Series) Size Controlling Apple Rootstocks

Ten trees each of Royal Gala and Honeycrisp on eight rootstocks (PI.80, M.26 EMLA, V.1, V.3, M.9 Nic. 29, Pajam 2, CG.16, M.9 T337, M.9 EMLA, Bud.9) were planted in a sandy silt-loam soil in 2002 at the Horticultural Experiment Station, Simcoe, Ontario. Trees were spaced 2.5 m within and 5.0 m between rows (800 trees/hectare; 323 trees/acre). Trees were trained to a vertical axe central leader system and were trickle irrigated. Results after seven years of production indicate the following:
Honeycrisp (Figure 2)

• Eight rootstocks (Bud.9, M.9 EMLA, M.9 T337, CG.16, V.3, Pajam 2, and M.9 Nic29) were significantly smaller than M.26 EMLA. PI.80, also known as Supporter 4, and V.1 were statistically similar in size to M.26 EMLA.
• Tree size (trunk cross-sectional area) ranged from 62 to 110% relative to M.26E with Bud.9 being the smallest
• Cumulative yields after 7 years were not statistically different among rootstocks. However, Bud.9, M.9 EMLA, and CG.16 were among the most yield efficient while PI.80 significantly less yield efficient.
• Cumulative yield efficiency has been significantly lower on PI.80 and M.26 EMLA in comparison with the other rootstocks
• In other Honeycrisp research conducted at the Horticultural Experiment Station, Simcoe, trees on M.26 or equivalent vigour have been preferred over M.9 because Honeycrisp is a very weak grown cultivar. However, when grafted on M.26, Honeycrisp grafted unions have had a tendency to break.

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Royal Gala (Figure 3):

• Eight rootstocks (Bud.9, M.9 EMLA, M.9 T337, M.9 Nic.29, V.3) were significantly smaller than M.26 EMLA. PI.80, CG.16, V.1 Pajam 2, and MM.106 were all statistically similar in size to M.26 EMLA.
• Tree size (trunk cross-sectional area) ranged from 64 to 104% relative to M.26E with Bud.9 being the smallest
• Cumulative yields after 7 years were statistically different among rootstocks. V.1, CG.16, Pajam 2, and V.3 had the highest cumulative yields. The following rootstocks had statistically lower yield efficiencies over five years of production: M.9 EMLA, M.26 EMLA, Pl.80, MM.106
  M.9 T337 had 30% tree mortality over the life of the experiment. This was higher than any other rootstock.

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Concluding Remarks

Apple producers have a wide selection of rootstocks to choose from. The decision of rootstock selection should be based on a number of factors including growing region (hardiness zone), soil fertility, orchard system, cultivar, tree spacing, and prevalence of disease. Our results indicate that M.9 and M.26 remain very productive size-controlling stocks with many positive attributes. Their primary weakness however is their high susceptibility to fire blight and lack of cold hardiness. Furthermore, M.9 T337, a common Dutch M.9 rootstock clone sold in North America over the past decade appears to be inferior to other M.9 clones based on the three studies above. The Vineland series rootstocks and Bud. 9, in particular, may help to address these concerns. When more vigour is required for weak growing cultivars such as Honeycrisp, consider a more vigorous rootstock that M.9, particularly in less fertile or sandy soils. More Information on the Vineland rootstocks can be obtained at www.plant.uoguelph.ca/treefruit.

Acknowledgements
The research was supported, in part, by the University of Guelph and the Ontario Ministry of Agriculture, Food and Rural Affairs Sustainable Production Systems and the Ontario Apple Growers' Association. The mission of the Sustainable Production Systems Program is to enhance the long-term global competitiveness and sustainability of Ontario's agriculture and food system through excellence in research