Large Trees – Heavy Pruning

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Staghorning

Tree Thinning / Removal

As avocado orchards age and grow, space within the orchard may become congested. A solution to this particular scenario may involve quite heavy canopy management. The two techniques outlined below include staghorning and tree thinning / removal.

Staghorning

Procedure:

Involves cutting trees back to a stump above the graft union and allowing them to re-grow. Carried out to rejuvenate older crowded orchards. Can also be used to top-work trees over to another variety.

Staghorning of large trees

 

 

Costs Involved:

Costs range from $22 – 55/tree ($4400 – 11,000/ha based on 200 trees/ha) this includes cutting down trees, chipping/mulching of limbs and painting of stumps.

Trees are cut down to a stump using either chainsaws or mechanical pruning saws. Mechanical saws can be used to reduce tree height and width (saws can handle branches up to 10-15cm diameter). Larger limbs are then cut down using chainsaws. A forestry flail mulcher mounted on an excavator arm can also be used to bring trees down.

Smaller branches can be mulched using slashing equipment. Larger branches can be mulched (tractor operated forestry mulching equipment can handle branches up to 25cm diameter), chipped (commercial operators available @ $235/hr) or removed from the orchard.

Considerations:

Timing

Trees are staghorned after harvest. In subtropical climates, trees are staghorned between June-August and in cool temperate climates between October-December.

Height of the Stump

Cut trees back to a stump of about 1m high but ensure that the cut is made above the graft union. Trees staghorned higher can become too large before they come back into production.

Trees cut back to a height of 1m and stumps painted to protect against sunburn.

Sunburn Protection

Exposed branches and stumps should be immediately painted with white plastic paint to prevent sunburn. Diluting the paint with water so it can be sprayed onto the exposed branches will reduce the time taken to apply protection. Adding a copper fungicide treatment (e.g. copper oxychloride) may assist in controlling disease at the pruned surface. Other preparations including bentonite clay and calcium carbonate sprays can also provide sunburn protection.

 

This tree was staghorned too high - red lines indicate where cuts should have been made.
This tree was staghorned too high – red lines indicate where cuts should have been made.

Time Out of Production

Trees can be out of production for 2-4 years after staghorning. To maintain cash flow, sections of the orchard can be staghorned at intervals so at least part of the orchard is always in production.

Replant Versus Staghorning

Since trees can be out of production for 2-4 years after staghorning, some growers prefer to replace older trees with new plantings utilising new rootstocks. Fallowing is recommended (eg. a sorghum cover crop) for Phytophthora control. In some situations, tree removal can pose a disease risk. Ensure that all stumps,sticks and roots down to a size of 1cm diameter are removed from the orchard and allow time for smaller sticks and roots to rot away prior to planting particularly in those orchards adjacent to rainforest and wet sclerophyll forests to prevent infection by the wood-rotting fungus Phellinus noxius. Replanting in these infested sites is not advised as the fungus can survive in root debris for several years.

Staghorning Alternate Rows or Alternate Trees Within a Row

Not recommended. For the stump to regrow effectively, adequate sunlight is required. Remaining trees tend to fill in the space provided and reduce the amount of light reaching the staghorned tree or row.

Tree Health at Time of Stumping

It is recommended that trees should be injected with phosphonate 4-6 weeks prior to staghorning to assist in Phytophthora control.

Nurse Branches

If trees are unhealthy, leave a nurse branch to support root growth. The nurse branch can also produce fruit thereby maintaining some production in a staghorned block. This method is also used when top-working to other varieties.

Staghorned tree with nurse branch.
Staghorned tree with nurse branch.                                  

Regrowth Management

It is important to manage the regrowth from staghorned trees. In this example the number of branches should have been reduced to ensure light penetration into the centre of the tree.

A staghorned tree requiring regrowth management.
A staghorned tree requiring regrowth management.

 

Selectively prune or mechanically prune regrowth to manage tree size as described in Section: Young Trees – Maintenance pruning. Strong vigorous shoots or water shoots can be removed or cut back to lateral branches.

Plant Growth Regulators

At the date of publication both SUNNY® (active ingredient uniconazole-P) and AuSTAR® (active ingredient paclobutrazol) were registered for use in avocado, however check the status of the registrations before planning an application. Also note that AuSTAR® cannot be used when there is mature fruit on the tree.

Trials in subtropical environments indicate that foliar application of SUNNY® (Active constituent: 50g/L uniconazole-P) and AuSTAR® (Active constituent: 250g/L paclobutrazol) at a rate of 5-10L/1000L to the summer and autumn growth flush can reduce shoot growth, increase flowering and yield in staghorned trees. New growth was treated when shoots are 50-100mm in length.

Do not apply plant growth regulators to trees with low vigour, under stress or showing symptoms of Phytophthora root rot.

Tree Thinning / Removal

Procedure:

Alternate rows of trees within a row are removed as orchards begin to crowd.

Orchard beginning to crowd
Orchard beginning to crowd

 

Alternate rows are removed to improve access and light penetration into the orchard.
Alternate rows are removed to improve access and light penetration into the orchard.

Removal of trees on the diagonal in square plantings may involve a change in row direction and this can pose difficulties with respect to irrigation lines.
Removal of trees on the diagonal in square plantings may involve a change in row direction and this can pose difficulties with respect to irrigation lines.

Costs involved

Costs range from $22 – 55/tree ($4400 – 11,000/ha based on 200 trees/ha) this includes cutting down trees, chipping/mulching of limbs and removal of stumps.

Trees are cut down using either chainsaws or mechanical pruners. Mechanical saws can be used to reduce tree height and width (saws can handle branches up to 10-15cm diameter). Larger limbs are then cut down using chainsaws. A forestry flail mulcher mounted on an excavator arm can also be used to bring trees down.

Smaller branches can be mulched using slashing equipment. Larger branches can be mulched (tractor operated forestry mulching equipment can handle branches up to 25cm diameter), chipped (commercial operators available @ $235/hr) or removed from the orchard.

Considerations

Remaining Trees

The remaining trees are pruned using selective limb removal or mechanical pruning techniques to prevent crowding.

Disease Risk

In some situations, tree removal can pose a disease risk. Ensure stumps and as many roots as possible are removed particularly in those orchards adjacent to rainforest and wet sclerophyll forests to prevent infection by the wood-rotting fungus Phellinus noxius.

Large Trees – Medium Pruning

Jump to these sections in the article:
Option 1: Pruning One-Third of the Tree

Option 2: Mechanical and Selective Pruning
Option 3: V-Shape Prune

Typically as orchards mature, in many regions canopy management needs to be undertaken to optimise and maintain orchard productivity. As the size of the avocado trees increase, canopy management may need to be conducted more vigorously to maintain the balance between orchard manageability and yield. This will most likely entail major limb removal. Three options have been recommended here for large avocado trees requiring major limb removal.

Option 1: Pruning One-Third of the Tree

Procedure:

Year 1: Manually prune one side of the tree after harvest. The eastern side is normally pruned first to minimise sunburn on the main branches. Regrowth on the eastern side is mechanically tip pruned during summer to encourage lateral branches. Water shoots or vigorous regrowth are removed.

Year 2: Centre limbs are removed after harvest. Water shoots or vigorous regrowth on the eastern side are cut back or removed.

In the first year the eastern side of the tree was removed. In the second year the centre branches will be removed as indicated by the red lines.

Year 3: Major limbs are cut back or removed from the other side (western side) of the tree. Water shoots or vigorous regrowth are cut back or removed.

In the third year major limbs on the western side of the tree were removed.

Costs Involved:

Costs range from $16.50 – 27.50/tree/year ($3300 – 5500/ha based on 200 trees/ha).

Branches are removed or cut back using chainsaws. Smaller branches can be mulched using slashing equipment. Larger branches can be mulched (tractor operated forestry mulching equipment can handle branches up to 25cm diameter); chipped (commercial operators available @ $230/hr) or removed from the orchard.

Considerations:

Pruning Time

Major limb removal is carried out after harvest. In those areas where trees carry fruit all year round, major limb removal can commence in an ‘off’ year to minimise crop losses.

Regrowth is mechanically tip pruned during summer to encourage lateral branches. Water shoots or vigorous regrowth are removed or cut back to lateral branches.

Sunburn Protection

Some sunburn on higher branches can be tolerated as these limbs are removed in the second or third year. However the trunk and lower branch stubs should be protected from sunburn using white acrylic paint, clay preparations or calcium carbonate sprays.

Diluting the paint with water so it can be sprayed onto the exposed branches will reduce the time taken to apply protection. Adding a copper fungicide treatment (e.g. copper oxychloride) may assist in controlling disease at the pruned surface.

Plant Growth Regulators

At the date of publication both Sunny® (active ingredient uniconazole-P) and AuSTAR® (active ingredient paclobutrazol) were registered for use in avocado, however check the status of the registrations before planning an application. Also note that AuSTAR® cannot be used when there is mature fruit on the tree.

Foliar application of SUNNY® (Active constituent: 50g/L uniconazole-P) at a rate of 5-10L/1000L to the summer and autumn growth flush can reduce shoot growth, and may increase flowering and yield on the pruned side. New growth is treated when shoots are 50-100mm in length. Note withholding period: Do not harvest fruit for 14 days after application.

Do not apply plant growth regulators to trees with low vigour, under stress or showing symptoms of Phytophthora root rot.

Option 2: Mechanical And Selective Pruning

Procedure:

Year 1: Trees are mechanically pruned on one side after harvest at an angle of 0-5° from the vertical. Branches are cut 1-1.5m from the trunk. Follow-up selective hand pruning is carried out to remove bare and exposed branches. Trees are mechanically tip pruned in summer to cut back regrowth.

Year 2: Trees are mechanically topped after harvest at an angle of 55-60° from the vertical. Tree height reduced to 4.5-5m. Regrowth on the side pruned in the first year also mechanically tip pruned at an angle of 15-22° after harvest. Trees are tip pruned on both pruned faces in summer to cut back regrowth.

Year 3 or 4: The other side of the tree is mechanically pruned after harvest at an angle of 15-22° from the vertical. Follow-up selective hand pruning is carried out to remove bare and exposed branches. Regrowth on the side pruned in the first year also mechanically tip pruned at an angle of 15-22° after harvest. Trees are tip pruned at 15-22° on both sides in summer to cut back regrowth.

Costs involved:

Costs range from $12 – 17.50/tree/year ($2400 – 3500/ha based on 200 trees/ha).

Trees are cut back using mechanical saws. Pruning contractors are available at rates of $175-235/hr. Mechanical saws can cut branches up to 10-15cm diameter. Bare branches are cut back using chainsaws. Trees are mechanically tip pruned in summer to cut back regrowth.

Smaller branches can be mulched using slashing equipment. Larger branches can be mulched (tractor operated forestry mulching equipment can handle branches up to 25cm diameter), chipped (commercial operators available @ $235/hr) or removed from the orchard.

Considerations:

Pruning Time

Pruning is carried out after harvest. In those areas where trees carry fruit all year this strategy can be done in an ‘off’ year to minimise crop losses prior to the ’on’ year’s flowering. Regrowth is tip pruned during summer to cut back the spring growth and encourage lateral branches.

Sunburn Protection

Exposed branches should be protected from sunburn using a white acrylic paint, clay preparations or calcium carbonate sprays. Diluting the paint with water so it can be sprayed onto the exposed branches will reduce the time taken to apply protection. Adding a copper fungicide treatment (e.g. copper oxychloride) may assist in controlling disease at the pruned surface.

When to Prune the Other Side

The other side of the tree can be pruned when the regrowth from the first pruning has come back into production. This can take 2-3 years depending on timing of the initial pruning and location.

Plant Growth Regulators

At the date of publication both SUNNY® (active ingredient uniconazole-P) and AuSTAR® (active ingredient paclobutrazol) were registered for use in avocado, however check the status of the registrations before planning an application. Also note that AuSTAR® cannot be used when there is mature fruit on the tree.

Foliar application of SUNNY® (Active constituent: 50g/L uniconazole-P) at a rate of 5-10L/1000L can be applied to the regrowth following the summer prune to reduce the amount of growth and increase flowering in the spring. New growth is treated when shoots are 50-100mm in length.

Do not apply plant growth regulators to trees with low vigour, under stress or showing symptoms of Phytophthora root rot.

Option 3: V-Shape Prune

 

Procedure:

Year 1: The eastern side of one row and the western side of the adjacent row are removed after harvest. More than 50% of the tree is removed. Minimal pruning to maintain access occurs in the next inter-row. The procedure is continued throughout the block. Regrowth on the pruned sides can be tip pruned in summer to encourage lateral branches. Water shoots or vigorous regrowth are removed.

Year 2 or 3: Regrowth on the pruned side is tip pruned in summer to encourage lateral branches. Water shoots or vigorous regrowth are removed. The other side of the tree is removed when the regrowth from the initial pruning comes into production. This can take 2-3 years depending on timing of the initial pruning and location.

Crowded orchard prior to pruning.
Trees pruned after harvest.
Trees six months after pruning.

Orchard access maintained in unpruned sides

Costs involved:

Costs range from $16.50 – 22.00/tree ($3300 – 4400/ha based on 200 trees/ha) this includes cutting down trees, mulching of limbs and applying sunburn protection. Chain saws are used to cut back the larger branches. Operations can be made quicker by using mechanical saws to remove the majority of branches. Mechanical pruning contractors are available at rates of $175-235/hr. Mechanical saws can cut branches up to 10-15cm diameter. Bare branches are cut back using chainsaws.

Smaller branches can be mulched using slashing equipment. Larger branches can be mulched (tractor operated forestry mulching equipment can handle branches up to 25cm in diameter), chipped (commercial operators available @ $235/hr) or removed from the orchard.

Considerations:

Timing

V-shape pruning is carried out after harvest. In those areas where trees carry fruit all year round major limb removal can be done in an ‘off’ year to minimise crop losses prior to the ’on’ year’s flowering. Regrowth can be tip pruned during summer to encourage lateral branches. Water shoots or vigorous regrowth are removed or cut back to lateral branches.

Removal of Over-Hanging Branches

These branches shade areas where regrowth is required. Reduced sunlight causes the regrowth to become elongated (‘shoots looking for light’).

Sunburn Protection

Some sunburn on higher branches can be tolerated as these limbs are removed in the second or third year. However the trunk and lower branch stubs should be protected from sunburn using a white acrylic paint, clay preparations or calcium carbonate sprays. Diluting the paint with water so it can be sprayed onto the exposed branches will reduce the time taken to apply protection. Adding a copper fungicide treatment (e.g. copper oxychloride) may assist in controlling disease at the pruned surface.

When to Prune the Other Side

The other side of the tree can be removed when the regrowth from the first pruning has come back into production. This can take 2-3 years.

Plant Growth Regulators

At the date of publication both SUNNY® (active ingredient uniconazole-P) and AuSTAR® (active ingredient paclobutrazol) were registered for use in avocado, however check the status of the registrations before planning an application. Also note that AuSTAR® cannot be used when there is mature fruit on the tree.

Foliar application of SUNNY® (Active constituent: 50g/L uniconazole-P) at a rate of 5-10L/1000L to the summer and autumn growth flush can reduce shoot growth and may increase flowering and yield on the pruned side. New growth is treated when shoots are 50-100mm in length. Note withholding period: Do not harvest fruit for 14 days after application.

Do not apply plant growth regulators to trees with low vigour, under stress or showing symptoms of Phytophthora root rot.

Large Trees – Light Pruning

Jump to these sections in the article:
Selective Limb Removal

Mechanical Pruning

Strategies used will depend on tree size and extent of orchard crowding. Techniques include selective limb removal, selective and mechanical pruning, major limb removal, staghorning, tree thinning/removal and plant growth regulator application.

Selective Limb Removal

Procedure:

These strategies involve removing individual limbs to reduce tree height and width and to open up canopies for light penetration, improved spray penetration and cherry-picker access.                           

Selective limb removal in large trees. Arrows indicate where branches have been removed.

Costs Involved:

Selective limb removal costs $11-22 /tree (depending on tree size or $2200-4400/ha based on 200 trees/ha).

Branches are removed or cut back using chainsaws. Smaller branches can be mulched using slashing equipment. Larger branches can be mulched (tractor operated forestry mulching equipment can handle branches up to 25cm diameter); chipped (commercial operators available @ $235/hr) or removed from the orchard.

Considerations:

Selection of Branches

• Reduce the number of main branches to 2-4 depending on tree and row spacing.
• Remove or cut back the dominant branches to reduce tree height. Removing the tallest branch will also improve light penetration and stimulate growth inside the tree.
• Remove or cut back branches that grow into neighbouring trees and the inter-row space to maintain orchard access.

• Remove poorly positioned branches such as overlapping (create a dense canopy) and crossing over branches (branches that grow from one side of the tree to the other) to improve light penetration, prevent dieback of internal shoots and improve spray penetration and picker access.
• Selective pruning is carried out each year to maintain tree size and ensure light penetration into the tree.

Where to Cut Branches

Branches should be cut back flush to the trunk or main lateral branch to minimise regrowth and promote rapid healing of the wound.

Pruning Time

Pruning is carried out immediately after harvest or during flowering. In locations where two crops are present on the tree, selective limb removal may commence in an ‘off’ year to minimise crop losses.

To avoid excessive regrowth (due to a light crop load), cut branches in late autumn/early winter. Pruning at this time can also minimise potential sunburn damage to exposed limbs and fruit.

Sunburn Protection

Any exposed branches should be protected from sunburn using white acrylic paint. Diluting the paint with water so it can be sprayed onto the exposed branches will reduce the time taken to apply protection. Other preparations including bentonite clay and calcium carbonate sprays can also provide sunburn protection. Adding a copper fungicide treatment (e.g. copper oxychloride) may assist in controlling disease at the pruned surface.

Mechanical Pruning

Procedure:

Trees are either pruned to a pyramid or ‘barn’ shape using mechanical saws to reduce tree size, maintain orchard access and maximise light penetration in the orchard. Trees are normally pruned after harvest and again in summer.

Mechanical pruning of an orchard in practice.

Costs Involved:

After harvest pruning including dragging out, cutting up and mulching of branches: $2.75 – 8.25/tree ($550 – 1650/ha based on 200 trees/ha).

Summer pruning including mulching: $1.80 – 2.35/tree ($350 – 470/ha based on 200 trees/ha).

Mechanical saws are used to reduce tree height and width and to maintain tree shape. Pruning contractors are available at rates of $175-235/hr. Large trees can take 3-4 passes each side to bring trees back to the desired size. Mechanical saws can cut branches up to 10-15cm diameter. Summer pruning involves a light trimming to cut back the spring growth flush.

Smaller branches can be mulched using slashing equipment. Larger branches can be mulched (tractor operated forestry mulching equipment can handle branches up to 25cm diameter); chipped (commercial operators available @ $235/hr) or removed from the orchard.

Considerations:

Pruning Time

Mechanical pruning is normally carried out twice a year – after harvest and during the summer. In warmer subtropical production areas the major shaping prune occurs after harvest in autumn and winter. Pruning too soon after harvest whilst the weather is still warm can encourage excessive regrowth at the time of early fruit set and may affect fruit retention. It is better to wait till late autumn or winter when temperatures are cooler. Once the tree shape has been established, a summer prune to cut back the spring growth flush can be implemented in the presence of fruit, it must be done well before the summer flush appears. The summer prune is made about 10-30cm outside of the post-harvest pruning face. The timing of this pruning can influence the level of flowering the following spring.

In southern production areas where the harvest occurs during spring and summer, mechanical pruning after harvest will remove developing fruit (two crops are carried on the tree at these locations) and expose fruit and limbs to sunburn. Mechanical pruning may be implemented in late autumn/early winter during an ‘off’ year to minimise crop loss.

Pruning Angle and Shape

Trees are either pruned to form a pyramid (Christmas tree shape) with the sides pruned at an angle of 15-22º from the vertical or to form a ‘barn’ shape with the sides pruned at an angle of 10-15° and the tops pruned at an angle of 60°.

Tree shapes after mechanical pruning.

 

Large Hass trees before pruning.

Large Hass trees after mechanical pruning                     

Other Pruning Options

For a major reshaping of trees, alternate sides can be mechanically pruned in consecutive years so to retain one side of the tree in production.

Selective Hand Pruning

Shortening or removal of bare and exposed branches (particularly after the after the initial ‘hard’ pruning) may be necessary to avoid vigorous regrowth at the pruning surface.

Bare branches should be cut back to avoid this undesirable regrowth.

Regular mechanical pruning can also result in a dense canopy wall at the pruning surface. Selective pruning to open up ‘windows’ may be required to improve light penetration and increase the efficiency of spraying and harvesting operations.

Plant Growth Regulators

At the date of publication both SUNNY® (active ingredient uniconazole-P) and AuSTAR® (active ingredient paclobutrazol) were registered for use in avocado, however check the status of the registrations before planning an application. Also note that AuSTAR® cannot be used when there is mature fruit on the tree.

Foliar application of SUNNY® (Active constituent: 50g/L uniconazole-P) at mid-bloom (when 50% of the flowers have opened) at a rate of 5-10L/1000L can be used to enhance fruit shape, increase fruit size and reduce the length of the spring growth flush. SUNNY® at a rate of 5-10L/1000L can also be applied to the regrowth following the summer prune to reduce the amount of growth and increase flowering in the spring. New growth is treated when shoots are 50-100mm in length. In South Africa two sprays two-three weeks apart at 2-3L/1000L have been used to control this regrowth. Note withholding period: Do not harvest fruit for 14 days after application.

Foliar application of AuSTAR® (Active constituent: 250g/L paclobutrazol) when trees are in full flower at a rate of 7L/1000L can be used to control vegetative growth. AuSTAR® can also be applied at fruit set at a rate of 5L/1000L to control fruit drop. AuSTAR® cannot be applied when mature fruit are on the tree. Note withholding period: Do not harvest fruit for three months after application.

Do not apply plant growth regulators to trees with low vigour, under stress or showing symptoms of Phytophthora root rot.

Young Trees – Maintenance Pruning

Jump to these sections in the article:
Selective Limb Removal

Mechanical Pruning
Branch Scoring

The objective of these pruning strategies is to maintain the trees at the desired size and shape as long as possible to prevent crowding while maintaining fruit quality and yield. These strategies can commence in the second or third year after planting. Strategies include selective limb removal, selective and mechanical pruning, branch-scoring and plant growth regulator application.

Selective limb removal

Procedure:

These strategies involve removing or cutting back individual branches to maintain tree size and inter-row access and to open up canopies for improved light and spray penetration and picker access.

Lower and centre branches were removed in this four year old Hass tree.

Costs involved:

Branches are removed or cut back using pruning or chain saws (up to 5 minutes/tree, labour @ $22/hr: based on 200 trees/ha = $366/ha).

Small branches are mulched using slashing equipment (tractor and slasher @ $66/hr).

Considerations:

Selection of branches

Reduce the number of main branches to 2-4 depending on tree and row spacing.

Remove or cut back the tallest branch. In young trees, height may not be a problem however the most dominant branch can be cut back to a major lateral branch. Removing the tallest branch will also improve light penetration into the tree.

Remove or cut back branches that protrude into the inter-row space and affect orchard access.

Remove poorly positioned branches such as overlapping (create a dense canopy) and crossing over branches (branches that grow from one side of the tree to the other) to improve light penetration, prevent dieback of internal shoots and improve spray penetration and picker access.

Remove branches on the north to north-east side of the tree to improve light penetration into the tree.

Low branches are removed (skirted) to avoid fruit contact on the ground, maintain sprinkler efficiency and allow access for trunk injection for Phytophthora control. However in harsh environments that experience heat waves such as the Tristate and Dimbulah in North Queensland, it is often better to keep low branches in place to shade the root zone and help maintain mulch in place.

Where to cut branches

Branches should be cut back flush to the trunk or main lateral branch to minimise regrowth and promote rapid healing of the wound.

Pruning time

Pruning is best done immediately after harvest or during flowering. In locations where two crops are present on the tree, selective limb removal may commence in an “off” year to minimise crop losses.

To avoid excessive regrowth (due to a light crop load) cut branches in late autumn/early winter. Pruning at this time can also minimise potential sunburn damage to exposed limbs and fruit.

Sunburn protection

Any branches newly exposed to sun as a result of pruning should be protected from sunburn using white acrylic paint. The more horizontal the limbs the more susceptible they are to sunburn.

Diluting the paint with water so it can be sprayed onto the exposed branches will reduce the time taken to apply protection. Other preparations including bentonite clay and calcium carbonate sprays can also provide sunburn protection. Adding a copper fungicide treatment (e.g. copper oxychloride) may assist in controlling disease at the pruned surface.

Mechanical pruning

Procedure:

Trees are pruned to form a hedgerow using mechanical saws. For maximum light penetration into the orchard a north-south orientation is preferred and trees are pruned to a pyramid shape or a “barn” shape to a height of up to 6m. There are several variations on the pruning angle being used.

Five year old Shepard trees mechanically pruned to a “barn” shape.

Costs involved:

Mechanical pruning costs typically range from $0.90 – 1.75/tree ($175 – 350/ha based on 200 trees/ha).

Mechanical saws are used to reduce tree height and width and to maintain tree shape. Pruning contractors are available at rates of $175-230/hr.

Small branches can be mulched using slashing equipment (tractor and slasher @ $60/hr).

Considerations:

When to start pruning

Trees can be initially shaped in the year prior to the first commercial production. In young trees, shoots are lightly tip pruned mechanically.

Pruning time

Mechanical pruning is normally carried out twice a year – after harvest and during the summer.

The post-harvest pruning establishes the shape of the tree. Trees are pruned prior to flowering; at early flowering or during winter – when trees are in a non-vegetative phase. Pruning too early after harvest (during a vegetative growth phase) can encourage excessive regrowth at the time of early fruit set.

Once the tree shape has been established a light summer prune to cut back the spring growth flush can be implemented in the presence of fruit. The summer prune is made about 10-30cm outside of the post-harvest pruning face. Timing of the summer prune is important and can affect the level of flowering on the regrowth the following spring. For example: in subtropical climates pruning mid-late summer reduces the percentage of shoots to flower.

Mechanical tip pruning of the spring flush during the summer prune. Arrows show pruning cuts.               

Pruning angle and shape

Trees are either pruned to form a pyramid (Christmas tree shape) with the sides pruned at an angle of 15-22º from the vertical, or to form a “barn” shape with the sides pruned at an angle of 10-15° and the tops pruned at an angle of 60°.

 

Tree shapes after mechanical pruning.

 

Five year old Hass trees mechanically pruned to a “pyramid” shape.
Four year old Hass trees mechanically pruned to a “barn” shape.

Selective hand pruning

Shortening or removing bare and exposed branches may be necessary to avoid vigorous regrowth at the pruning surface.

Bare branches should be cut back to avoid this undesirable regrowth.

Regular mechanical pruning can result in a dense canopy wall at the pruning surface. Selective pruning (branch removal) to open up ‘windows’ may be required for light penetration into the tree and to improve the efficiency of spraying and harvesting.

Growing environment

Mechanical pruning is more suited to warm subtropical environments where fruit is harvested before the next flowering occurs. In climates where two crops are carried on the tree implementing a mechanical pruning strategy after harvest is difficult without removing developing fruit and exposing fruit to sunburn. Mechanical pruning (tip pruning) in late autumn has been used in South-West Western Australia on young trees up to six years. Flowers can develop below the cut surface on small branches.

Flowers developing behind the cut surface in trees tip pruned in late autumn.

Plant Growth Regulators

At the date of publication both SUNNY® (active ingredient uniconazole-P) and AuSTAR® (active ingredient paclobutrazol) were registered for use in avocado, however, check the status of the registrations before planning an application. Also note that AuSTAR® cannot be used when there is mature fruit on the tree.

Foliar application of SUNNY® (Active constituent: 50g/L uniconazole-P) at mid-bloom (when 50% of the flowers have opened) at a rate of 5-10L/1000L can be used to enhance fruit shape, increase fruit size and reduce the length of the spring growth flush. SUNNY® at a rate of 5-10L/1000L can also be applied to the regrowth following the summer prune to reduce the amount of growth and increase flowering in the spring. New growth is treated when shoots are 50-100mm in length. Note withholding period: Do not harvest fruit for 14 days after application.

Foliar application of AuSTAR® (Active constituent: 250g/L paclobutrazol) when trees are in full flower at a rate of 7L/1000L can be used to control vegetative growth. AuSTAR® can also be applied at fruit set at a rate of 5L/1000L to control fruit drop. Do not apply when mature fruit are on the tree. Note withholding period: Do not harvest fruit for three months after application.

Do not apply plant growth regulators to trees with low vigour, under stress or showing symptoms of Phytophthora root rot.

Branch scoring

Procedure:

Involves cutting a groove around the branch to sever the phloem using a knife or pruning saw. The aim is to increase flowering and fruiting on the scored branch. When successfully carried out, the wound will produce callus tissue and eventually heal, thereby restoring the normal function of the branch. Healing of the wound can occur within 2-4 months depending on the growing conditions.

Note: This technique is still experimental under Australian conditions.

Costs involved:

Branches are scored using a knife or pruning saw (2 minutes/tree, labour @ $22/hr: based on 200 trees/ha = $146/ha).

Three year old Hass tree scored in April showing increased flowering on the scored branch.

Considerations:

Selection Of branches

Rule of thumb: no more than one-third of the branches should be scored on any one tree in any one year. Either the tallest branch to reduce tree height or a centre branch to open up the canopy is selected for scoring. The score cut is usually made above lateral branches. In young trees only 1-2 branches are scored to prevent root starvation. After the fruit is harvested the branch can be removed as part of a selective limb removal management strategy.

1-2 branches are scored in young trees, this branch will be removed after harvest.
This branch was scored incorrectly and no effect on flowering was observed. On the side of the branch where healing of the wound did not occur, the groove was not cut deep enough.

Timing

Branches are scored in autumn to reduce vegetative growth and increase flowering and fruit set the following spring.

Cincturing versus scoring

Cincturing involves the removal of a strip of bark about 10mm wide. Scoring involves cutting a groove no more than 2-3mm wide. The wound in scored branches can heal within 2-4 months thereby restoring the normal function of the branch. The wider cincture wound can take several months to heal and can have detrimental effects on branch health including small fruit, reduced leaf growth and even death.

Growing conditions

Growers in southern production areas (Hinterland of Mid North Coast NSW, Central Coast NSW, Tristate and South-West Western Australia) have been trialling this scoring technique to reduce vegetative growth and increase flowering and fruit set in the selected branch. However in some cases, yellowing of leaves and leaf drop has occurred resulting in sunburnt fruit. Nitrogen management may prevent this occurring.

Excessive leaf drop on the scored branch can expose fruit and branches to sunburn.

Training Young Trees

Jump to these sections in the article:
Tip Pruning

Central Leader Pruning

The objective of these pruning strategies is to establish a tree shape suited to the orchard layout and canopy management strategy to be implemented during the life of the orchard. These procedures are normally carried out for the first 12-18 months after planting. Two main strategies used are tip pruning and central leader pruning.

Tip Pruning

Procedure:

Growing tips are pinched out or cut back to stimulate the growth of side shoots. Trees may be tipped 2-4 times during the first 12-18 months depending on the growing conditions.

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Shoot tip removed.      figure%204%20side%20shoots%20stimulated%20v21

   Side shoots stimulated.

Costs Involved:

Shoots tips are removed or cut back (1 minute/tree, labour @ $22/hr: based on 200 trees/ha = $74/ha).

Considerations:

Tree Shape

Tip pruning produces a bushier, more rounded tree suited to wider tree spacing. Using this method, 3-4 major branches can be established early which assists in developing a tree shape suited to a selective limb removal strategy later on.

Central Leader Pruning

Procedure:

This strategy involves the removal or cutting back of rapidly growing side branches to produce a strong central leader.

This is achieved by:
1. Removing side branches that are more than half the thickness of the central leader
2. Cutting back side branches that are more than one-third the thickness of the central leader
3. Keeping side branches that are less than one-third of the thickness of the central leader.

Trees may be pruned 2-4 times during the first 12-18 months depending on the growing conditions.

Pruning

Costs Involved:

Small shoots/branches are removed or cut back (2 minutes/tree, labour @ $22/hr: based on 200 trees/ha = $146/ha).

Considerations:

In The Nursery

If this strategy is going to be adopted it is important that trees develop a strong central leader in the nursery. The main shoot should not be tipped.

Pruning Strategy

Young trees are trained to form a central leader with lateral branches along its length. This strategy develops a pyramid shaped tree better suited to a hedgerow pruning system later on. Higher planting densities can be achieved and maintained using this strategy.

Variety Planted

Central leader pruning is more suited to upright varieties such as Reed and Lamb Hass. However this strategy is being used on Hass overseas (e.g. South Africa, Chile, California and New Zealand).

Growing Environment

A single central leader may be difficult to maintain under the vigorous growing conditions of a warm subtropical climate. Developing a tree shape with 2-3 upright leaders may be more attainable.

Orchard Establishment

Jump to these sections in the article:
Tree Spacing

Row Direction
Planting Configuration

Orchard layout depends on:
• Cultivar (upright varieties such as Reed and Lamb Hass can be planted closer than other varieties such as Hass and Shepard)
• Amount of land available and slope of the site
• Growing conditions (soil and environment)
• Machinery access (safe machinery operation e.g. tractors, cherrypickers and pruners)
• Canopy management system to be used during the life of the orchard.

Tree Spacing

Low Density Plantings
e.g. 12 x 12m; 10 x 10m and 10 x 8m (69 – 125 trees/hectare):
• Provide lower early returns per hectare
• Lower establishment costs and less pruning and tree removal costs in later years
• Results in large trees with increased harvesting costs, reduced picker safety and more difficult to spray for effective pest and disease control.

Medium Density Plantings
e.g. 8 x 7m; 9 x 6m; 10 x 5m; 7 x 7m and 8 x 5m (178 – 250 trees/hectare):
• Tree width and height will need to be controlled as crowding occurs to maintain orchard access, allow effective pest and disease control and improve light penetration.

High and Ultra-High Density Plantings
e.g. 8 x 4m; 6 x 5m and 7 x 3m (312 – 476 trees/hectare) and 6 x 3m; 3 x 3m and 2.25 x 2.25m (555 – 1975 trees/hectare):
• Provide higher early returns per hectare
• Higher establishment costs
• Tree width and height will need to be controlled or trees removed as crowding occurs
• Ultra high densities are more suitable for upright varieties such as Reed and Lamb Hass and in environmental conditions where tree vigour is less.

Row Direction

To maximise light penetration into the orchard it is recommended that trees are planted in rows running in a north-south direction. However, row direction needs to suit the slope of the site. On slopes greater than 15% rows should run up and down the slope to allow safe machinery operation. Ensure appropriate erosion control measures are in place in orchards where rows are running up and down the slope.

Planting Configuration

Trees are planted either on a square or rectangle configuration. A rectangle configuration where trees are spaced closer within the row than between the rows is favoured and allows the creation of hedgerows.

Trees can also be planted in a regular or alternate arrangement.

The regular planting arrangement is more common, particularly in hedgerow systems. However, an alternate planting arrangement may allow better light penetration into the orchard.

Biosecurity

Effective orchard biosecurity practices are essential to reducing the threat of new pests impacting on your orchard. In 2011 and 2020, Avocados Australia and Plant Health Australia released two biosecurity planning documents critical to ensuring the protection of Australian avocado production against exotic pests. These were a revised Industry Biosecurity Plan for the Avocado Industry (V3) and the first Orchard Biosecurity Manual for the Avocado Industry (V1.0). (Links to these documents can be found under the Related Resources at the top of the page.)

IMPORTANT NOTE: Biosecurity measures also have a part to play, and must be considered, when moving produce interstate or overseas. In the case of moving produce interstate it is important to seek prior advice from Australia’s state and territory domestic plant quarantine regulators on the plant entry quarantine conditions and requirements. The contact details for all the state and territory quarantine regulators can be found HERE. On that page you will also be able to access the plant health, quarantine and biosecurity manuals for each state and territory.

What is Biosecurity?

Biosecurity is about the protection of livelihoods, lifestyles and the natural environment, which could be harmed by the introduction of new pests. Biosecurity is a national priority, implemented off-shore, at the border and on-farm. Biosecurity is essential for your business.

Australia’s geographic isolation has meant that we have relatively few of the pests that affect plant industries overseas. Freedom from these exotic pests is a vital part of the future profitability and sustainability of our plant industries. Biosecurity allows us to preserve existing trade opportunities and provide evidence to support new market negotiations.

High Priority Plant Pests – Resources

Avocados Australia’s Biosecurity Project, a High Priority Plant Pest (HPPP) Analysis was completed that identifies the pests that present the greatest risks for Australia’s avocado growers. Detailed “Pest Assessments” of each pest have been produced that outline such things as host information, pathways, invasive capacity, treatment and management options.

You can access all of the Pest Assessments documents from the BPR Library HERE. Look under “Biosecurity”. The twenty (20) HPPPs that were identified appear below:

Plant Health Australia developed an “Exotic Pest Identification & Surveillance Guide for Tropical Horticulture“, you can access a free electronic copy HERE.

The 2nd edition of the “Avocado Problem Solver Field Guide” is a comprehensive resource that can assist you with identifying exotic pests. Order a copy using this online form HERE.

Current Incursions – Update

Varroa destructor (Varroa Mite) – Management Plan – Click Link

Orchard Biosecurity Manual

The Orchard Biosecurity Manual is a practical document designed to assist you in protecting your property and the avocado industry from new and invasive pests. By implementing the recommended measures in your day-to-day operations, you will improve your own biosecurity and that of your region, while minimising produce losses and unnecessary costs. Orchard biosecurity is a set of measures designed to protect a property from the entry and spread of pests.

Orchard biosecurity is your responsibility, and that of every person visiting or working on your property.
Through the implementation of orchard biosecurity measures, growers play a key role in protecting the Australian avocado industry from exotic pests. If a new pest becomes established in your orchard, it will affect your business through increased orchard costs (for monitoring, cultural practices, additional chemical use and labour to apply them), reduced productivity (yield and/or quality reductions) or loss of markets. Early detection and immediate reporting increase the chance of effective and efficient eradication.

If you have found a suspected exotic plant pest, the following general precautions should be taken immediately to contain the pest and protect other parts of your orchard:
• Mark the location of the pest or symptoms and limit access to the area for both people and equipment.
• Wash hands, clothes and boots that have been in contact with affected plant material or soil.
• Restrict operations in the area while waiting for the identification of the suspected exotic pest.
If you see anything unusual, call the Exotic Plant Pest Hotline – 1800 084 881.

Click here to access & action the Orchard Biosecurity Manual document (3,162KB)

Click here to access the Industry Biosecurity Plan document (1,583KB)

Click here to watch Biosecurity Bite videos and learn more about Australian Biosecurity and Trade, Market Access requests, Import risk analysis, Border restrictions, Exotic plant pests, Export processes and your role in Australian Biosecurity.

For more information visit the Plant Health Australia website – www.planthealthaustralia.com.au

Some states have a General Biosecurity Obligation, which means everyone must take reasonable steps to prevent or minimise biosecurity risks. Find out about your obligations under your state or territory’s biosecurity act:

Postharvest Management of Anthracnose

Important: Management of the disease requires an integrated preventive programme both in the field and postharvest.

Minimise delays in the delivery chain wherever possible; the longer the period between harvest and consumption, the worse the disease will become.

Temperature management

Before packing

Keep harvested fruit out of direct sunlight to prevent overheating and remove field heat as quickly as possible. Follow postharvest temperature protocols established to minimise postharvest disease development. Maintenance of the cool chain is essential.

If picking, packing and cooling within 24 hours of harvest is possible, pre-cool fruit to 16°C to remove field heat.

If picking, packing and cooling within 24 hours is not possible, pre-cool fruit to 7°C.
Precooling should use a high humidity room.

If there is no packing facility at the farm, accumulated bins of harvested fruit need to be transported in batches. Harvested fruit must be kept as cool as possible. This may mean leaving bins in the orchard under shade or transferring them to a cool, shaded holding facility. Time at the farm should be minimised to enabling packing (and cooling) within 24 hours of harvest. If the weather is hot (>25°C) and bins are to be transported on an open truck, it is better to delay transport until temperatures drop and the sun is not intense. Overnight transfer from farm to packing facility can provide some cooling effect if temperatures are low.

Forced-air cooling is strongly recommended if warm fruit needs to be cooled before packing. Forced-air cooling systems can remove heat up to 10 times faster than room-cooling. These systems also avoid condensation forming on the fruit, which can occur due to temperature gradients inside bins during room-cooling.

After packing

Below are general guidelines, for more details please refer to the industry’s “Avocado ripening manual”.

  • Before ripening: green mature fruit at 5°C for Hass and 7°C for other varieties
  • During ripening with ethylene: 16 to 20°C
  • After ripening: 5°C for Hass and 7°C for other varieties

Lower storage temperatures than these may cause chilling damage. Temperature has a critical effect on anthracnose development during fruit ripening. Once fruit starts to ripen, temperatures of 24°C and above will greatly accelerate development of the disease.

Postharvest fungicide treatment

Important: Postharvest treatment is not a substitute for field spraying – both are necessary.

Treat fruit within 24 hours of harvest with a registered postharvest fungicide to suppress the latent infections in the skin and to help prevent the development of postharvest rots. Ensure that label instructions are followed, these include ensuring that fruit are wet with the fungicide for at least 30 seconds.

In 2022 there were two postharvest fungicides registered for use on avocado, prochloraz (e.g. Sportak®) and fludioxonil + azoxystrobin (e.g. Graduate A+®)

Note: If azoxystrobin was the last field spray, the fludioxonil + azoxystrobin postharvest treatment should not be used (this is part of the anti-resistance strategy).

If exporting the fruit, check that the importing country approves of the postharvest fungicide treatment and that Maximum Residue Levels (MRLs) and withholding periods (WHPs) are not exceeded. A useful tool is the Avocados Australia’s Maximum Residue Limits APP.

Identification of Anthracnose

 

Symptoms mainly occur on ripe fruit even though infection takes place much earlier. Symptoms are rarely seen on leaves and stems.

Fruit – pre-harvest

Two types of pre-harvest symptoms may occur but are not generally visible on Hass, only on green skin varieties.

(a) Large circular brown spots form as a secondary infection around puncture marks on the skin (e.g. caused by feeding and/or egg-laying insects, hail damage and wind rub). The spots darken with age, centres become sunken, and in moist conditions pinkish spore masses may form on the spots. About 90% of affected fruit drop before harvest.

        anthracnose lesions

Well-developed lesions on a green skin variety showing pink spore masses.

(b) Small disfiguring spots less than 5 mm in diameter develop on the skin, often around the lenticels (pores on the fruit skin). Sometimes a teardrop pattern of spots can be observed as the infection runs from the top of the fruit to the base (spores are carried down the fruit by water during extended rainfall periods). The spots rarely penetrate the ripened flesh but downgrade the cosmetic appearance of the fruit. They do not cause fruit drop.

anthracnose%20p41

Small discoloured spots develop around the lenticels.

Fruit postharvest

This is the most widespread and important type of anthracnose symptom. The disease starts in the field but remains dormant until ripening. It is responsible for serious commercial losses and undermines consumer confidence in avocado. As the fruit begins to ripen, small, light brown and circular spots develop on the skin (this is hard to detect on Hass). These enlarge and darken with time, centres become sunken, and the flesh underneath breaks down. The rot penetrates deeply into the flesh in a hemispherical pattern. Under moist conditions pinkish spore masses may develop on the skin spots.

anthracnose%20p61

Advanced external symptoms on a green skin variety showing brown/pink spore masses.

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Left: Characteristic hemispherical shape of about eight separate anthracnose infections penetrating deep into the flesh.
Right: Anthracnose causes severe internal damage to fruit. Damage is characterised by a flesh rot that expands hemispherically from the point of infection.

Stems and mummified fruit

Pink spore masses may be present on dead twigs and mummified fruit in the tree canopy.

Fruit that are missed at harvest can mummify and become a source of disease spores, so can dead twigs and branches. The pink/brown spore masses can be seen in these examples (Photo – Elizabeth Dann, UQ)

Leaves

Symptoms on leaves are rare and generally only appear after prolonged wet and humid weather. They are more likely to be seen in nurseries and glasshouses. Spots are large and tan-coloured, with dark brown margins. Under very humid conditions pinkish spore masses may develop on the spots.

Anthracnose spots on leaves.

About Anthracnose

 

Pathogens

It can be caused by a number of different Colletotrichum species.

  • In the warmer parts of Australia, the most common species belong to the Colletotrichum gloeosporioides complex (in particular C. alienum, C. fructicola, C. siamense and C. perseae)
  • In the cooler regions, Colletotrichum fiorinae (within the C. acutatum complex) is important.


Infection

Large numbers of fungal spores are produced in the tree on dead twigs, leaves and fruit and can often be seen as pink spore masses. The spores are spread through the orchard during periods of wet weather, overhead irrigation and heavy dews. Studies have shown that the most important source of fungal spores for the development of anthracnose is from dead plant material entangled in the tree canopy rather than from leaf litter beneath trees.

After landing on undamaged fruit surfaces, spores germinate and penetrate the skin with an infection peg. Spore germination requires free surface water from rainfall, irrigation or dew for a number of hours, although some studies indicate that germination can also occur at relative humidity above 95% (even when no free moisture is visible). After the infection peg penetrates the fruit skin (usually within 48 hours after the spore first lands on the fruit surface) the fungal infection is then resistant to adverse wet or dry weather conditions. The infection remains dormant until the fruit begins to ripen. Throughout this stage, there are no visible signs of disease, but the latent infection is maintained. Fruit is susceptible to infection through all stages of development.

Anthracnose disease cycle

Where field infection occurs through damaged areas on the fruit such as insect stings, the process is slightly different. Here, infection occurs, but continues to develop through a ‘localised’ pseudo-ripening effect around the damaged tissues. As a result, the fungus completes its development through to localised rotting and the production of spore masses for a new cycle of infection.

Climate

Fruit in both warm and cool growing regions are susceptible to anthracnose infection. The Colletotrichum gloeosporioides species complex favour warm (more than 15°C), humid conditions (such as those found in summer rainfall regions of the subtropical and tropical regions of eastern Australia) but Colletotrichum fiorinae favours cooler regions such as southwest Western Australia. In southwest Western Australia with its Mediterranean climate, the critical periods are autumn, winter and spring when the rain occurs. Although anthracnose infections are uncommon in the Tristate region, the use of overhead misting for cooling the canopy and out of season rain does result in its occurrence.

Any sort of damage to the fruit skin, such as hail, wind rub and insect damage will make the fruit more susceptible to anthracnose infection.

Avocado variety

Varieties differ in their susceptibility to the disease, for example Fuerte, Maluma and Wurtz are more susceptible than Hass, and Hass is more susceptible than Sharwil and Shepard. However, all varieties require an effective fungicide spray program.

While it is unusual to find visible symptoms on Hass fruit in the orchard, poorly protected fruit will develop anthracnose symptoms upon ripening. Unfortunately for the retailer and consumer this is masked by the thick dark skin and is only discovered when fruit are cut in preparation for consumption.

Rootstock

Rootstock plays a major role in the susceptibility of the scion variety to anthracnose; however little research has been done comparing relative susceptibility across a range of different rootstocks. We do know that Hass trees on Velvick rootstock (a West Indian x Guatemalan rootstock) have less anthracnose in their fruit than Hass grafted to Duke 6 (Mexican race rootstock). The lower incidence on Velvick rootstocks is attributed to two factors:

a) Higher levels of dienes (naturally occurring antifungal compounds) produced by the rootstock which transfers to the fruit.

b) Higher uptake and transfer of calcium to fruit on trees with Velvick rootstocks, compared to Mexican rootstocks.