Kiwi - Rootstock or Free Plants: How to Choose?

Introduction

Choosing the right rootstock is a crucial step in the success of a kiwifruit plantation. The rootstock, which is the lower part of the grafted tree, including the root system, is the base upon which the scion is implanted. This choice has a direct impact on several essential aspects of kiwifruit cultivation, such as the plant's vigor, disease resistance, and productivity.

In its quest to revive production, the French kiwifruit industry is taking a keen interest in introducing new rootstocks. Although growing kiwifruit on their own roots ("free-rooted" plants) is still practiced, this method has limitations in the face of current challenges, particularly in terms of disease resistance and adaptation to different soil types.

Comparison between Free-Root Plants and Rootstock Plants for Kiwi Cultivation

Frankish Plants

• Definition: A kiwi plant is a kiwi plant grown on its own roots, without grafting. The above-ground part of the plant is the same as the underground part.
• Root System: Frankish plants develop a large, taproot system, which allows them to anchor themselves firmly in the soil and reach deep for water.
• Vigor and Resistance: Grafted plants are generally more vigorous and more drought-resistant than grafted plants. They also have good longevity.
• Adaptation to Terrain: They often adapt better to local soil and climate conditions.
• Suitable Soil Types:
  • Frankish plants prefer deep, rich, neutral and fresh, well-drained soils.
  • Avoid calcareous soils and those that retain too much water.
• Fruit Set: Fruit production is slower with hardy plants, generally 4 to 5 years after planting.
• Pollination: It is necessary to provide at least one male plant for every 5 to 7 female plants to ensure good pollination.
• Use: The method is more suitable for non-commercial crops.

Plants on Rootstock

• Definition: A rootstock plant is a kiwi plant whose aerial part (the scion) is grafted onto a specific root system (the rootstock).
• Root System: The root system is determined by the rootstock used, which can provide better anchorage and nutrient absorption.
• Vigor and Size: Rootstock helps control the vigor and size of the tree, which can make orchard management easier.
• Adaptation to Difficult Soils: Rootstocks allow plants to adapt to difficult soils, including calcareous soils, soils with drainage problems, poor or shallow soils. The choice of rootstock must be made based on soil analysis.
• Disease and Pest Resistance: Rootstocks offer better resistance to soil-borne pathogens and specific kiwifruit diseases. For example:
  • Bounty 71 rootstock is tolerant to Psa ( Pseudomonas syringae pv. actinidiae ).
  • Roki rootstock has better resistance to moria.
• Fruit Set: Fruit set is faster with grafted plants, generally 2 to 3 years after planting.
• Varietal Compatibility: The rootstock must be compatible with the kiwi variety being grown to ensure a good union between the rootstock and the scion.
• Use: The use of rootstocks is more suitable for commercial production.

How to Choose a Suitable Rootstock?

There is no single rootstock that is perfect for all situations. The choice of rootstock must be made based on the specifics of each farm and the grower's objectives. Therefore, it is essential to consider several factors when selecting the most appropriate rootstock.

• Soil type is a determining factor:
  • For calcareous soils, it is important to choose rootstocks that are resistant to them. Kiwis generally prefer slightly acidic soils, with a pH of 6 to 6.5. Therefore, soils with an active lime content above 4-5% should be avoided.
  • Heavy, clayey soils, prone to compaction and moisture, can be improved with the use of SAV1 rootstock, which has good resistance to water pressure and stagnation. Bounty 71 rootstock is also suitable for heavy, poorly drained soils.
• Local climatic conditions must be taken into account:
  • Sensitivity to frost should be considered, as it can damage buds at temperatures as low as -5°C.
  • Kiwis are sensitive to drought, so you need to choose a rootstock that is resistant to this water stress, or plan an irrigation system.
• Resistance to specific kiwi diseases is an essential criterion:
  • Bounty 71 rootstock shows commercial tolerance to Psa ( Pseudomonas syringae pv. actinidiae ), a bacterium that is very damaging to kiwi crops.
  • Yellow and red kiwifruit are more susceptible to PSA than others and are usually grown under cover.
• Desired vigor for plants:

  The rootstock influences the size of the tree and its growth rate. Kiwis are vines that require support.

• Compatibility with the kiwi variety being grown is crucial