olive

Fruit production: How to manage olive trees

What happens after the olive orchard has been planted?

Irrigation is essential in modern olive orchards. The yield and fruit quality of table olives (for pickling) and oil olives are improved significantly by well-managed irrigation. But too much, or poorly scheduled, irrigation can lead to all sorts of problems, including loss of trees to asphyxia and root rot.

To ensure optimal tree performance, the application of water should match water use, and water losses through over-irrigating should be eliminated. The daily water requirement of olives may be less than for other fruit crops, but the period for which irrigation is needed is longer, because the olive is evergreen and harvesting takes place in autumn.

A young tree needs 20 litres of water every week during the first growing season. As a rough guide, a mature orchard in full production needs 720 mm of irrigation per annum, less the rainfall and the peak demand is in summer and early autumn.

The critical period for irrigating is from pit hardening to about a week before harvesting.

IRRIGATION OPTIONS

Double or single line drippers or microsprinkler systems, are used successfully by local growers. The range of apparatus available today helps optimise irrigation scheduling.

It is recommended that a rough estimate be made of water requirements, based on evapotranspiration and using historical evaporation figures for the area as well as crop factors. The evapotranspiration is then reduced by a factor depending on the tree size and age.

The application rate can be optimised according to observation and measurement. The simplest measurements are made by collecting soil samples at regular intervals, after irrigation, and determining water content by feel.

Increasing levels of sophistication, accuracy and convenience are possible with increasingly sophisticated and costly instruments; for example, capacitance probes which give accurate continuous indications of soil moisture at various levels.

More sophisticated systems, such as well-managed computerised fertigation based on real time tree requirements, facilitate reduced water usage and fertiliser usage; much more efficient, easier and cheaper nutrient application; reduced leaching and pollution; and improved production and fruit quality.

This higher initial investment results in ongoing reductions in costs, plus higher income, giving improved profitability for the lifetime of the orchard.

Also read: Irrigation – which system should you use?

NUTRITION/FERTILISING

  • Olives, like other fruit trees, require macronutrient elements such as nitrogen (N), potassium (K), phosphorus (P), calcium (Ca), magnesium (Mg) and sulphur (S).
  • Trace elements(required in much smaller quantities) are, in general order of importance, boron (B), zinc (Zn), iron (Fe), manganese (Mn), copper (Cu) and molybdenum (Mo).

Managing nitrogen correctly is critical for the development and correct formation of young trees and for maintaining the balance between growth and cropping. Nitrogen, in the form of ammonium sulphate or LAN, is utilised throughout the growing season, and is usually applied to young trees every month in spring and summer.

It is recommended that nitrogen be applied regularly and in smaller doses to prevent the formation of vigorous infertile watershoots and excessive vegetative growth caused by single large applications.

  • Be careful of injudicious applications of nitrogen in summer.
  • Potassium (eg potassium sulphate) is usually applied in spring and autumn.
  • Under local conditions, replenishments of N, K, B and Zn are important in early spring since cropping removes nutrients from the system.
  • Nutrient elements are applied according to rates determined through leaf and soil analysis and adjusted according to expected crop levels, otherwise imbalances can occur.

Maintaining soil acidity at the right pH level (5.5 to 6.5) is critical for facilitating the optimal uptake of nutrients. Soil pH is determined during soil analysis (which should be done for each orchard at least every 3 years) and rectified by annual applications of lime, according to recommendations.

Leaf analysis should be done every year in January. Foliar sprays using LB urea (200 g/100 litres) plus a wetting agent in late winter and early spring have proved beneficial since root activity in cold soil is very low.

3 regular soluble boron sprays (100 g/100 litres) during flower development in spring have also been found to be beneficial, as boron applied to the soil does not translocate or transport easily or quickly to the flowers, where it is needed.

A urea spray just after pit hardening has been found to improve fruit growth and quality. Urea at 100 g/100 litres is often incorporated with fungicides and pesticide sprays. Apply organic material to ensure soil health and adequate soil carbon levels.

Beneficial micro-organisms are essential for root function and to combat pathogens. Localised application of organic material, under the irrigation dripper, is cost effective. Mulching has additional benefits in conserving soil moisture and modulating soil temperatures.

WEED MANAGEMENT

  • Weeds compete with the tree for available moisture, nutrients and oxygen in the soil.
  • This can be reduced by regular mowing or by spraying with herbicides.
  • Glyphosate and fluazifop (both post-emergent) are currently registered for use on olives in South Africa.
  • Glyphosate should not be sprayed on any part of the tree, and the stems of young trees should be protected using tree guards.
  • Chemical weed killers have to be used with care, since the long-term effects on tree performance and the environment appear to be suspect.

A relatively thick mulch layer of high lignin material in the tree row helps suppress weed growth and provides a habitat for earthworms and arthropods, which redistribute organic material and nutrients into the soil, and improve water infiltration and gas exchange at the same time, stimulating beneficial fine root development and microbe population.

PRUNING AND TRAINING

  • During the first year in the orchard, pruning is kept to a minimum.
  • Only those branches impeding others or growing too near the soil surface are removed.
  • There are a number of approaches to pruning and training olive trees.
  • In an extensive system, 3 to 6 main scaffold branches are selected to form a semi-open vase.
  • All infertile watershoots and most other upright growth in the centre of the tree are removed to improve the interception of light throughout the tree.
  • Take care not to expose the scaffolds to the risk of sunburn damage.
  • In a more intensive system, a single leader tree is planted and this main leader is then staked and trained upright.
  • Lateral branching is encouraged over the entire length of the leader from a height of about 40 cm upwards.
  • Lateral branches should have a diameter of less than a third of that of the main leader, and grow at a wide angle.
  • Any upright growth on the laterals should be removed regularly.

In time, once these laterals have borne fruit, they are renewed on a constant, rotational basis by removing 1 in every 4 depleted bearing units, while another 25%, regularly spaced laterals are stubbed to one node to form future bearing units.

All pruning is done with secateurs, and saws are not required until the end of the cycle when, after about 20 years, the entire central leader can be renewed just above the soil line.

This integrated intensive system can only work successfully with suitable cultivars in a trellised high-density system, with trees planted 1.2 m to 1.5 m apart in rows 3.5 m to 4 m apart, under well-managed, regular, specific fertigation and pruning.

OLIVE PESTS

The main olive pests in South Africa are:

  • The yellow and black striped olive beetle, whose bright yellow larvae eat and tunnel into leaves, especially on young trees, destroying new growth.
  • The olive lace bug (tingid) which sucks sap from the leaves, especially where growth is dense, and so causes tiny yellow dots on the leaves, which later become completely chlorotic and die.
  • The olive fly stings the fruit and lays its eggs which then hatch and destroy the fruit as the larvae burrow through the flesh.

FUNGAL DISEASES

  • Anthracnose (Gloeosporium) causes rapid fruit rot and cankers on shoots.
  • Peacock spot, results in sooty spots, later surrounded by a yellow halo, and subsequent yellowing of leaves, later resulting in leaf drop and death of shoots.
  • Leaf blotch (Cercospora) symptoms are irregular blotchy yellowing of leaves, necrosis of tips and edges of the leaf, and sooty spots on the underside of the leaf.
  • Various soil-borne root diseases (Phytophthora, Verticillium, Phoma, etc) occur as a result of poor irrigation scheduling.

Also read: An introduction to olive farming

  • This article was written by Carlos Costa and first appeared in Farming SA.

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