Nutrition and soils for M.9 rootstock in high-density plantings
Young apple orchards on M.9 need good nutrition for strong spring growth and good bud break, and do not compete well for water and nutrients.
With trees on M.9, light or sandy soils need to be mulched.
For best results, apple trees on M.9 need deep, fertile, and well-drained soils, and the best orchard management.
Where unfavourable climates and soils prevent tree growth extending further into the season, and to maximise the performance of high density apple orchards on M.9, growers may have to:
- Ridge the shallow top soil
- Install drip irrigation (preferably subsurface and pulsating)
- Fertigate frequently
- Mulch the soil
- Cover the trees with net and/or install overhead cooling
- Plant well-feathered trees closely enough so they can quickly fill their allotted spaces and reach their maximum permissible height.
What if these measures cannot be implemented or are not cost-effective?
With woolly apple aphid continuing to be a major apple pest and with only limited control by chemicals, predators and parasites, resistant rootstocks with a range of dwarfing and semi-dwarfing growth habits, will provide a long-term solution.
One immediate solution is to stick with MM.106 because:
- Trees on this rootstock are precocious and with heavy crops, remain calm and manageable.
- MM.106 has performed very well in harsh climates for more than 60 years.
- MM.106 is the best known semi-dwarfing rootstock for apple trees in Australia, the USA, Canada, South Africa, South America and New Zealand due to early and high production, good tree growth, and good anchorage.
- Trees on MM.106 grow shoots for longer than on M.9. The initial vigour of trees on MM.106, which also extends further into the season than trees on M.9, quickly fills the trees’ spaces. Once cropped, trees grow more slowly and remain highly productive.
- MM.106 has excellent nursery characteristics. It grafts easily with high success. M.9 tends to be temperamental and unless the grafting is done well, up to 30 per cent of grafts can fail.
- MM.106 is resistant to woolly apple aphid.
- MM.106 is highly susceptible to Phytophthora. However, it has also been found in other rootstocks including M.9 if the soil gets wet enough. Good drainage is essential and rows should be ridged. Pre and post-harvest foliar sprays of phosphorus acid products (e.g. Agri-fos) will reduce the risk of infection.
- MM.106 tends to have a more dwarfing effect on Granny Smith trees than on other varieties.
Where conditions are less suited to M.9
For orchards in climates and soils less suited to the dwarfing M.9 rootstock, the vigorous MM.106 rootstock remains one of the growers’ best options.
If you are worried about excess vigour in your trees, remember that excess vigour is controlled more easily than is weak growth; and that you can use numerous tools, such as the growth regulators Prohexadione-calcium (sold as Regalis® or Apogee®) and Paclobutrazol (sold as Payback®), which keep trees calm, improve skin blush, and reduce the cost of summer pruning.
See this article in Tree Fruit August 2017
