Sooty blotch is widespread throughout the wet subtropical and coastal regions. It is a superficial blemish but reduces the market value of fruit.
Identification:
It can occur on all plant parts, especially in the shaded parts of the tree. Spores germinate readily on plant parts during warm, rainy weather to produce uneven smoky, sooty superficial blotches on the surface. Unlike sooty mould it does not need honeydew produced by insects to become established and the dark staining of sooty blotch cannot be rubbed off.
Sooty blotch gives green branches a dirty grey appearance. Sooty mould can look similar but is usually darker and thicker.
An uneven black superficial blemish develops on the skin surface. It is hard to rub off. Often the symptoms are streaked.
Treatment/prevention:
The program of copper sprays applied for anthracnose should adequately control sooty blotch, provided good spray coverage is maintained. Provide better ventilation to reduce dampness and humidity by pruning and shaping trees to keep the canopy open.
The soil-borne water mould Phytophthora cinnamomi.
General comments:
Tends to be isolated to a few trees here and there, for example, when irrigation sprinklers are placed too close to the trunk, the trunk has been damaged, or there has been a cyclone. It has occurred in Hass on Mexican race rootstocks in North Queensland following cyclones, where very wet conditions were combined with physical damage to the tree trunk from flying debris.
Identification:
The canker appears as a dark brown water-soaked area on the bark. Healthy bark is normally a grey-brown colour. A white secretion of dried sap is usually evident within the affected area; however, the presence of the white secretion alone does not necessarily indicate trunk canker disease (chronic boron deficiency and injury to the trunk may produce the same symptom). After some time the affected section ‘sinks’ into the trunk.
Active trunk cankers bleed sap that dries to form a white powder.
Lesions, splits and sunken areas on the trunk from older infections.
Treatment/prevention:
Using a sharp knife, pare away all the diseased bark and wood and paint the area with a mixture of a copper fungicide and water-based paint. Alternatively, spray affected trunks with a registered phosphonate fungicide. Protect the trunk from physical damage to prevent the pathogen from gaining entry. Avoid continuous wetness of the trunk from sprinklers or from having mulch material against the trunk.
The fungus Colletotrichum gloesporioides, the same pathogen that causes anthracnose.
General comments
Pepper spot is observed on fruit in the field, unlike anthracnose, which mostly doesn’t develop until fruit is ripening. It is more likely to occur on fruit surfaces that are affected by sunburn, even mild sunburn, and in trees that are stressed (e.g. by drought or Phytophthora root rot). It is a cosmetic problem only affecting the fruit surface, but it downgrades the fruit. Pepper spot is known as ‘speckle’ in South Africa.
Identification
Numerous minute, shiny black, raised spots (0.1 mm to 0.5 mm in diameter) develop on the surface of the fruit, fruit stems and twigs, especially on the warm northern and western sides of trees. Symptoms appear from mid to late summer. Spots are particularly prevalent on Hass fruit. Sometimes ‘tear stain’ marks are formed on the fruit surface.
Tiny black raised spots develop on green twigs and fruit.
Pepper spot consists of hundreds of very small, raised, black and shiny spots on fruit, fruit stalks and leaf stems. At times they merge together to form black patches. Pepper spot is most likely to be found on mildly sunburnt parts of the fruit.
Sunburn predisposes fruit to pepper spot.
Treatment/prevention
Take steps to minimise exposure of fruit to sunburn; this includes keeping phytophthora root rot under control.
The fungicide spray program recommended for anthracnose control helps to control pepper spot.
The disease was identified in Australia in 1992 in several orchards on the Atherton Tableland of North Queensland, but the disease does not appear to have spread from there. Varieties vary in susceptibility, with Fuerte and Sharwil being far more susceptible than Hass and Shepard. It is a significant disease in South Africa where it is known as ‘black spot’. Fruit quality is reduced, becoming unsaleable. Spores are easily spread by wind and rain to initiate new infections.
Identification
The disease symptoms can appear on leaves, stems and fruit.
Leaves: Symptoms initially appear on the underside of leaves as small (1 to 5 mm), angular, light brown to purple spots surrounded by a yellow halo. The angular shape of the spots and their yellow halo distinguishes this disease from others. The same spots eventually become visible on the upper surface of the leaf. As they grow, individual spots can join to form larger lesions. Under humid conditions grey spore bodies may develop on the spots and can be seen with the aid of a hand lens.
Cercospora spots on the upper (left) and lower (right) surfaces of the leaf.
Close-ups of spots on the upper surface. Spots are angular and light brown to purple with a yellow halo.
Fruit: Spots are initially small, raised and black, becoming slightly sunken and cracked over time. Other fungal diseases such as anthracnose can invade the fruit through these cracked lesions.
Raised black spots become slightly sunken and cracked as the disease develops.
Skin sliced off the fruit shows the depth of the lesions.
Stems: Dark brown to black irregular lesions can develop on green twigs and fruit stems causing small fruit to fall off.
Treatment/prevention
Generally well-controlled by the fungicide spray program recommended for anthracnose control.
This disease has become more prevalent and widespread since the early 2000s. To date it has been found in orchards from the Atherton Tableland in North Queensland south to Northern New South Wales. It has a very wide range of host plants and has been found in a range of environments. It can survive for many years in roots and woody debris in the soil. The disease spreads from infected trees via root contact.
Identification
The most obvious symptom is a sudden whole-tree collapse caused by a girdling of the vascular system. Leaves suddenly wilt and turn brown but may remain attached to the tree for many weeks. The whole tree is affected and always dies. In some cases there is a slower deterioration in tree vigour and leaves become pale and drop prior to complete tree death. A brown crusty ‘stocking’ growing up the trunk from the root collar may also be present. Particles of soil and mulch are bound onto this ‘stocking’ at the soil level and the wood underneath is dead. The disease normally spreads sequentially along the row from the first tree that is infected. While Phytophthora cinnamomi only affects the white feeder roots, Phellinus noxius advances up the large roots all the way to the trunk, encrusting the roots with a blackened infection stocking. Brown root rot can be confused with waterlogging and severe frost damage. Verticillium wilt causes similar sudden death in the canopy but this is usually confined to a single branch and unless it is very young, the tree recovers.
Brown root rot causes the leaves of the entire canopy to wilt suddenly and then die within a few days.
The characteristic infection ‘stocking’ moving up the trunk.
The wood has been cut back to reveal a clear demarcation between the dead wood behind the ‘stocking’ on the left-hand side and the yet-to-be-affected trunk area on the right-hand side.
Brown root rot is transmitted along the tree row via root-to-root contact and all infected trees die. In this photo, infection has moved along the row from right to left. The apparently healthy tree on the left is likely to be infected already.
Treatment/prevention
In fields being prepared for avocado planting, remove from above and the below ground all sticks and roots bigger than one centimetre in thickness and allow enough time for smaller sticks to rot away before tree establishment. This disease is thought to be able to survive only in plant material. Where an infected tree has been positively identified, remove the whole tree as well as the tree on either side of it (even if they are not showing symptoms they are probably already infected) together with all wood and roots. This material should be burnt—do not chip and use as mulch. Install root barriers around the infected soil area to prevent roots from healthy trees coming into contact with infested woody material. No treatment for infected trees is currently available and it is difficult to re-establish new trees in the same planting site.
The fungus Calonectria ilicicola (formerly Cylindrocladium parasiticum).
General comments
A recently described root disease in Australia, which can cause severe damage to young trees and often leads to tree death.
Identification
The disease affects young trees before and after planting out. In milder cases the above-ground part of an infected tree may appear healthy but growth will be stunted. Roots show large areas of severe dark brown or black dead tissue, as well as smaller, brown lesions. The disease is favoured by high humidity, wet conditions, shading and high temperatures. The extent of the disease can be made worse by transplant shock and over-irrigation. Dieback in newly planted trees usually starts in the youngest tips and is accompanied by extensive root death. It can lead to large losses of trees in young avocado plantings.
Black root rot can be confused with Phytophthora cinnamomi root rot; however there is no evidence that black root rot kills older, more established trees. Phytophthora cinnamomi usually causes a slow decline with all-over canopy decline, whereas Calonectria ilicicola is more rapid. Feeder root death can also be caused by brown root rot and natural dieback at flowering time.
Dark brown or black feeder roots can be seen in young trees in the nursery or at planting out.
The youngest part of the tree is usually affected first (top) and sometimes the whole tree dies (bottom).
Treatment/prevention
Good nursery hygiene is essential. Try to avoid planting out during periods of wet weather, high humidity and high temperatures. Take care not to over-irrigate.
The bacterium Pectobacterium carotovorum (formerly Erwinia carotovora).
General comments
Bacterial soft rot does not occur often but can be a major problem when the right conditions arise.
Often seen after cyclones or other extended rain events that are accompanied by strong winds.
Physical damage to wet fruit e.g. caused by wind-driven debris or debris thrown up by machinery especially slashers, can lead to soft rot especially on low hanging fruit exposed to soil splash.
Most likely to be seen after harvest in fruit that was picked during rain and has some injury to the skin (e.g. anthracnose, insect damage or mechanical injury).
An infected fruit on the sorting belt can soon infect many more.
Widespread in Shepard fruit following Cyclone Larry (in North Queensland) due to the combination of wind damage to fruit and very wet conditions.
The disease inoculum can be splashed from the soil onto low-hanging fruit.
Identification
Fruit has a putrid smell.
Externally, the fruit initially has a darkened metallic sheen then the skin develops cracks as the disease progresses
Internally the flesh has soft, grey to black water-soaked areas of rot.
Green skin varieties such as Fuerte, Sharwil, Shepard and Reed are most susceptible, particularly when over-mature at harvest.
A metallic sheen is typical of early symptoms of bacterial soft rot.
As the disease advances the skin cracks and the flesh rots, emitting a putrid smell.
Symptoms develop progressively through the flesh and are accompanied by a putrid smell.
Treatment / Prevention
Manage pests effectively.
Use windbreaks to reduce fruit injury from wind rub.
Consider skirting to avoid having fruit close to the ground that are exposed to soil splash
Do not harvest during wet weather.
During wet weather, do not operate machinery in the orchard that could damage fruit.
Harvest fruit carefully to avoid skin injuries
In susceptible varieties consider clip picking rather than snap picking.
Do not use dip tanks to wash fruit – once a diseased fruit has passed through, there is potential for every subsequent fruit to be infected.
Considered a very minor disease with insignificant consequences, does not appear to damage the tree but will reduce photosynthetic capacity of affected leaves
Requires wet, humid, and shaded environments for infection
Under prolonged moist conditions the algae may cover the whole leaf
Identification
The algal spots are whitish to pale yellow and are most common on the upper leaf surfaces. They become reddish brown as they age due to spore development.
Algal spots can range from white to yellow to reddish brown (close-up on right).
Treatment / Prevention
Manage tree canopies to improve light penetration and ventilation.
The disease does not warrant spraying, however copper fungicides applied to control other diseases will reduce algal growth.
Crop cycle calendars have been prepared for each of the main production regions of Australia.
Please click on “Related Resources” above to view all Crop Cycle Calendars.
These calendars are intended to give the grower a guide to the main management practices, such as pest and disease control, due at different times in the seasonal crop cycle. Further detail about each practice can be found in other sections of the Best Practice Resource. Growers can use the calendars as memory joggers for what needs to be undertaken in the orchard at different times of the year. Please be aware that variations occur between seasons and between localities within each region so growers need to exercise judgment in following the timing suggested. Management activities should be linked to events in the crop cycle rather than to the month of the year.
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.
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.
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.
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
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.
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.
Active trunk cankers bleed sap that dries to form a white powder.
Lesions, splits and sunken areas on the trunk from older infections.
Tiny black raised spots develop on green twigs and fruit.
Pepper spot consists of hundreds of very small, raised, black and shiny spots on fruit, fruit stalks and leaf stems. At times they merge together to form black patches. Pepper spot is most likely to be found on mildly sunburnt parts of the fruit.
Sunburn predisposes fruit to pepper spot.
Close-ups of spots on the upper surface. Spots are angular and light brown to purple with a yellow halo.
Raised black spots become slightly sunken and cracked as the disease develops.
Skin sliced off the fruit shows the depth of the lesions.
Brown root rot causes the leaves of the entire canopy to wilt suddenly and then die within a few days.
The characteristic infection ‘stocking’ moving up the trunk.
The wood has been cut back to reveal a clear demarcation between the dead wood behind the ‘stocking’ on the left-hand side and the yet-to-be-affected trunk area on the right-hand side.
Brown root rot is transmitted along the tree row via root-to-root contact and all infected trees die. In this photo, infection has moved along the row from right to left. The apparently healthy tree on the left is likely to be infected already.
Dark brown or black feeder roots can be seen in young trees in the nursery or at planting out.
The youngest part of the tree is usually affected first (top) and sometimes the whole tree dies (bottom).
