Advocates for using the M9 or M26 rootstock for high-density apple plantings in southern mainland Australia, should think again.
With scorching temperatures in November and December 2012, and repeated temperatures around 40C in January, the outlook for good quality, non-sunburnt apples must be grim. And we are not even half-way into the summer!
Many orchardists planted apple trees on either M9 or M26 with the view to control vigour and increase precocity. They followed advice and recommendations from scientists, nurserymen and orchardists in countries that have cool climates, including Tasmania and New Zealand.
In Europe and the USA, dwarfing rootstocks have been, among other things, evaluated for cold hardiness, not if they can withstand high soil temperatures and high temperatures in the orchard.
Effect of high soil temperature
The optimum temperatures for root growth are in the range of 20C to 25C. At higher temperatures growth starts to decline, and above 35C new roots are no longer formed in apple and stone fruit trees.
This means that in areas with very hot summers root growth is limited especially in the upper soil layer, although drought is probably also involved.
Overseas experience
USA
It has been reported from the USA that trees on semi-dwarfing rootstocks (such as M7 and MM106) are more heat tolerant than trees on M9. Orchards on M9 are short-lived in very warm areas.
Israel
In Israel, rootstocks of the M and MM series are considered unsuitable because of the high soil temperatures in summer, the lack of adequate cold to break dormancy, or both.
South Africa
It is unfortunate that Australian orchardists take very little notice of the apple rootstocks being used in South Africa.
Evaluation of the M and MM rootstocks was done there many years ago. It was concluded that dwarfing rootstocks, which inhibit vigour, lead to an insufficient foliage cover—which resulted in an unacceptable degree of sunburn of fruit.
As a result of its dwarfing characteristics, these rootstocks could not be recommended for commercial orchards.
Tree growth & fruit quality
Insufficient leaf coverage and subsequent risk of sunburnt apples is partly due to the fact that bearing trees on M9 and M26 shut down earlier in the season than trees on M7 or MM106.
The small fibrous root systems of these dwarfing rootstocks, their sensitivity to high soil temperatures and dry soil conditions, and their extraordinary precocity may be factors contributing to the fruit quality problems in areas of Australia that experience very hot summer temperatures.
Netting, if you can afford it, and sunscreen materials may be of little help when temperatures reach 42C for five consecutive days or more. I have already seen in the Goulburn Valley, Victoria, sunburnt apples in November (not under net).
Better choices
Whether you believe in climate change or climate variability, tree vigour that allows you to grow a canopy with sufficient leaf coverage, such as with MM106, may be a better choice than M9 or M26 for areas with hot summers, and perhaps hotter summers to come.
Quince vs pear rootstocks for pears
As an adjunct, I question, why the Department of Primary Industries at Tatura is pursuing quince rootstocks as an alternative to Pyrus calleryana D6, when it has been well-documented, that survival of pear trees on quince rootstocks in hot summers is poor, although the Provence quince BA29 is slightly better than MA, MC or Adams in this respect.
Trees on pear rootstock (Pyrus communis), such as D6 perform better in warm conditions than quinces.
As well, in 2007 I documented that pear trees on D6 on Open Tatura in a Goulburn Valley demonstration orchard, outperformed other pear rootstocks, tree densities and training systems in respect of early yields. Yet, we are led to believe, that D6 is not suitable for high density planting.
For more information & photos, see Tree Fruit, January 2013
