Written by  Judy Tisdall and Bas van den Ende

Manage soils & water to control tree growth & increase productivity (part 10)

Chemical growth regulators are being used extensively in controlling excessive shoot growth and enhancing the production of deciduous fruit trees.

Growth regulators restrict roots (continued from last month)
There are three growth regulators or growth retardants for vigour control, Regalis®, Paclobutrazol and ethephon. Care needs to be taken in using these growth retardants. Strict adherence to recommendations is required to avoid damage to fruit trees and crop loss.
Growth retardants decrease growth, but have occasional negative side effects on fruit quality, and may perceive environmental or human health risks, and may not be registered for use on fruit trees in some countries.
Size-controlling rootstocks do not restrict roots
Size-controlling rootstocks, if available, do not control vigour. Grafting or budding to a size-controlling rootstock sacrifices the option for rapid vegetative growth.
However, size-controlling apple rootstocks have been used successfully to decrease vigour of the scion and keep apple trees to a manageable size.
To a lesser extent, size-controlling rootstocks have been developed for other fruit crops.
Many countries have or are currently breeding a range of size-controlling rootstocks which are better suited to a particular country’s needs. A few of these rootstocks may prove to be ideal, but many will either fail or adapt to a small niche location. It will take many years to prove rootstock adaptability to multiple conditions.
The roots of size-controlling rootstocks have slow rates of growth that are genetically determined, and, which limit the rate at which the fruit tree can grow above ground.
Even with vigorous roots, the soil limits the growth of the fruit trees above ground more than air does. Hence, you should manage your soil, not size-controlling rootstocks, to restrict roots.
Size-controlling rootstocks are not the panacea we have been led to believe. Comparative size differences between the currently planted rootstocks can also be affected by soil structure, nutrition and tree management.
Pear and peach trees on seedling rootstock, and even self-rooted fruit trees, have outperformed clonal rootstocks.
Another problem with size-controlling rootstocks is that excessive crop loads can stall the growth of young fruit trees.
Hormones such as auxin are normally moved from apical shoots to the root system to encourage root development. Pruning removes these apical shoots and as a result, decreases root development.
(Continued next month)

See this article in Tree Fruit May 2018