Field Management of Anthracnose

 

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

Revised orchard fungicide programme for anthracnose in avocado
Given the recent granting by APVMA of an Emergency Use Permit (PER94009) for Score® and Switch® fungicides, and resistance in Colletotrichum to strobilurin fungicides (e.g. azoxystrobin), a revised orchard fungicide program has been developed.

Revised orchard fungicide programme by Dr Elizabeth Dann, UQ – PDF HERE


Balanced nitrogen and calcium nutrition

There is a positive correlation between anthracnose susceptibility and fruit nitrogen levels, and a negative correlation between anthracnose and fruit calcium levels. The higher the nitrogen and the lower the calcium, the more severe the anthracnose. A nitrogen:calcium ratio in the fruit flesh of less than 30:1 is associated with more robust fruit and these are less susceptible to breakdown from fruit rots.

The only time that calcium can accumulate in the fruit is during the first 6 to 8 weeks after fruitset. This is the period in its development that the fruit has stomata. Calcium in the soil solution is taken up passively by avocado roots and moves through the tree in the transpiration stream, as water exits the fruit via its stomata calcium is deposited in the fruit. About 6 to 8 weeks after fruitset, stomata in the fruit skin cease to function, becoming lenticels, and the opportunity to accumulate calcium in the fruit is over. Adequate calcium in the soil solution, adequate soil moisture and a healthy root system are therefore essential for adequate fruit calcium.

Undertake regular leaf and soil analyses to keep nutrients, particularly calcium and nitrogen, in balance as this improves resistance of the fruit to infection. Excess nitrogen lowers the level of calcium in the fruit and makes it more susceptible to anthracnose.

Fruit on trees with high crop loads are less susceptible to anthracnose because of improved distribution of calcium into developing fruit instead of leaf.

If possible choose rootstocks that give elevated calcium levels in the fruit. For example Velvick provides this but Duke 6 does not.

Insect pest management

Control pests that compromise the fruit skin and thus provide entry points for anthracnose. These pests include fruit spotting bug and fruit fly.

Management of the canopy

Trees that are kept open through canopy management to allow better light penetration and easier picker access, will also improve ventilation to reduce dampness and humidity and result in better spray coverage. Skirting (pruning off lower limbs) will also assist the canopy to dry out after rain but it can be counterproductive to maintaining a hospitable (cool and moist) rootzone especially in regions that experience hot and dry conditions.

Dead wood and dead leaves tangled in the canopy act as a reservoir of fungal spores.

Hygiene

Keep the orchard clean; before flowering, prune out dead wood and remove infected fruit, dead leaves and twigs from the canopy that serve as sources of fungal spores. This can be carried out by the pickers.

Windbreaks

In exposed areas use windbreaks to reduce the incidence of wind rub. Damage caused by wind rub provides easy entry points for the fungi.


Orchard fungicide programme for anthracnose in avocado fruit

In order to protect it from anthracnose, fruit needs to carry a protective film of fungicide throughout the entire period it is on the tree. Think of it as keeping the entire fruit surface covered with a protective paint. Protective sprays should start at the end of flowering (copper fungicides can be phytotoxic to flowers) and continue until the fruit is harvested. Regular sprays are necessary to replace fungicide washed off by rain and because as the fruit grows it exposes new unprotected skin to infection. High-volume sprays are generally best to achieve thorough coverage.

Fungicides registered for this purpose are called ‘protectants’. At December 2023 registered protectant fungicide sprays in avocado include copper-based chemicals, the biofungicide Bacillus amyloliquefaciens strain QST 713 (Serenade Opti® Biofungicide, and Serenade Prime under Permit PER93515), azoxystrobin (e.g. Amistar®), fluopyram + trifloxystrobin (Luna Sensation®). Azoxystrobin and Luna Sensation may also have post-infection activity. Recently, testing has shown that after many years, the strobilurin fungicides (eg. azoxystrobin), may not be as effective at reducing anthracnose in fruit as populations of the causal fungi, Colletotrichum spp., have developed resistance to this group (see this article). Two fungicides, difenoconazole (Score®) and cyprodinil + fludioxonil (Switch®), have been granted use under Permit PER94009). These represent three new chemical groups for field application in avocado.

There are strict rules and limitations for the use of the post-infection fungicides to reduce the risk of resistance in the fungus and to extend the life of the fungicides. . They include only using in rotation with other registered fungicides that have a different mode of action, using no more than the allowable sprays per season, not using them consecutively etc. Also, the last pre-harvest field spray used cannot be azoxystrobin if fludioxonil + azoxystrobin (Graduate A+) is to be used as a post-harvest fungicide treatment.   It is extremely important to maintain good coverage of protectant (e.g. copper-based) fungicides, and this is achieved by regular sprays. A regular spray schedule from fruit set to harvest ensures that the fruit remains covered in a protective film as it grows. Use the shortest registered spray interval when conditions are particularly favourable for infection eg. wet weather, overhead misting in heatwaves, after windstorms, hail and in the presence of high inoculum levels. Expect to be applying a protective fungicide spray every 7 to 28 days depending on conditions. Irregular application of protectant fungicides will not be sufficient to adequately control anthracnose. The post-infection fungicides are useful after extended rainy periods if it has been difficult to keep up protectant coverage, and also pre-harvest.

Important: regularly check current registrations and permits and always follow the label or permit directions for use and withholding periods.

Note: Spraying phosphorous acid either at the same time as copper fungicides or when copper fungicide residues are present on the tree can sometimes result in leaf and fruit burn particularly if copper hydroxide is used and hot weather occurs.

Harvest management

Do not pick fruit when it is raining. Wet fruit is more turgid and therefore more sensitive to mechanical abrasion which will predispose it to infection. This is particularly important for Hass which has prominent skin nodules which are easily damaged.

During harvesting and packing, handle fruit carefully to avoid impact damage. Limit drop heights to less than 30 cm prior to packing. Ripe fruit can develop anthracnose at the site of impact damage.

During dry conditions, Hass can be snap picked but clip picking should be considered after wet weather especially if Plant Growth Regulators (PGRs) have been used, these conditions predispose the fruit to tearing at the stem which can lead to infection.

Canopy Management


Principles of Avocado Canopy Management

This video explains why canopy management is so important for avocado and outlines the main principles. However, a prescriptive approach cannot be provided since growing environments and management styles vary widely and no two orchards are ever the same. A canopy management plan needs to be worked out to suit each orchard.

Mainstream avocado canopy management options

The video below outlines some canopy management options for Hass and Shepard orchards planted at conventional spacings on the east coast of Australia where trees don’t have fruit on the tree all year. It complements the above video. Click on the image below to access the video.

The videos above were produced by DAF as part of project AV17005 “Avocado industry development and extension”.


Jump to these sections in the article:
Acknowledgements
Reasons for canopy management
Rules of thumb for tree size
When to prune?
Cropping Patterns

Acknowledgements

This publication formed part of project AV04008 and was funded using the Avocado Industry levy and funds from the Australian Government through Horticulture Innovation Australia Limited. All dollar values within this publication are estimates only provided as a relative guide of costs.

Thanks to all growers who provided information on their canopy management operations including: Agriexchange Pty Ltd; Avowest; Balmoral Orchards; GJ & MM Burch; Coastal Avocados; Delais Orchards; Donovan Family Investment Trust; CF Fechner; Gala Orchards; Hilltop Mareeba; Lavers Orchards; I & R Philpott; D & D Roche and Son; Simpson Farms Pty Ltd; Smerdon Enterprises Pty Ltd; Thistledome Farms; Toohey Farming and Yaldon Park. Simon Newett (Queensland Department of Agriculture and Fisheries) and Lachlan Donovan (Donovan Family Investment Trust) provided valuable comments and contributions in compiling this booklet.

Reasons for canopy management

• Optimise light interception and penetration into the orchard
• Maintain inter-row space for machinery access
• Reduce harvesting costs and improve picker safety
• Improve efficiency of spraying operations to control pests and diseases
• Maximise yields and fruit quality

Insufficient light due to crowding results in dieback of inner and lower branches creating large unproductive areas.

Large trees are more costly and more dangerous to harvest.

Rules of thumb for tree size

For optimum levels of sunlight to reach the tree canopy:

1. The height of the tree should not exceed 80% of the distance between tree rows.

Table 1   Relationship between tree height and distance between rows

2. Tree height should be less than 2.5 times the width of the canopy-free inter-row space (e.g. If requiring a 2m clearance for machinery access down the row, the maximum tree height should be 5m.)

3. No part of the tree canopy should be more than 2 to 2.5m from direct sunlight. Maximum canopy width should be 4 to 5m. Note: this is the ideal theoretical situation but is often impractical. In cool production areas with low vegetative vigour it can be achieved in very high-density plantings with upright varieties such as Reed and Lamb Hass.

When to prune?

Pruning time will depend on the growing environment. Knowing the cropping cycle is important in determining when trees are best pruned.

Cropping patterns

Shoot growth, flowering and fruiting cycles vary from region to region.

Less than 12 months from flowering to harvest:

In warm subtropical climates (e.g. North and Central Queensland and coastal areas of South-East Queensland) the crop can be harvested up to several months prior to next season’s flowering.

Table 2   Growth cycles for Shepard avocado in North Queensland
Table 3   Growth cycles for Shepard avocado in Central Queensland
Table 4   Growth cycles for Hass avocado in North Queensland
Table 5   Growth cycles for Hass avocado in Central Queensland

Table 6   Growth cycles for Hass avocado in coastal areas of South-East Queensland

In these environments pruning operations are normally done after harvest and prior to flowering. Follow up pruning to remove watershoots and to reduce the spring growth flush are often made in summer.

More than 12 months from flowering to harvest:

In cooler temperate climates (e.g. hinterland areas of Southern Queensland and Northern NSW, Central NSW) and Mediterranean climates (the Tristate region and south-west Western Australia) it is normal for the tree to carry two crops for a period of time. Mature fruit as well as the current season’s crop.

Table 7   Growth cycles for Hass avocado in hinterland areas of Southern Queensland
Table 8   Growth cycles for Hass avocado in Northern New South Wales and Mid North Coast New South Wales
Table 9   Growth cycles for Hass avocado in Central Coast New South Wales and Mid North Coast New South Wales hinterland
Table 10   Growth cycles for Hass avocado in the Tristate region
Table 11   Growth cycles for Hass in the Perth region in Western Australia
Table 12   Growth cycles for Hass in Bunbury and Bussellton region in Western Australia
Table 13   Growth cycles for Hass in south-west Western Australia

In these environments, pruning after harvest will not only remove flowers and developing fruit but can also expose this fruit to sunburn. Pruning (typically selective limb removal) is often carried out after the “on” year harvest and when trees are in an “off” year to minimise fruit loss. Mechanical tip pruning of young trees in late autumn has also been used in these environments.

Anthracnose


Key resources

 Revised orchard fungicide programme for anthracnose in avocado – Liz Dann (4 pages)
Given the recent granting by APVMA of an Emergency Use Permit (PER94009) for
Score® and Switch® fungicides, and resistance in Colletotrichum to strobilurin
fungicides (e.g. azoxystrobin), it is time to revise the orchard fungicide programme.

» Presentation: WA AvoSkills 2020 – ‘Diseases (other than Phytophthora) and their management in avocado’ – Liz Dann (33 slides)

» Presentation: Tamborine Northern Rivers Regional Forum 2022 – ‘Fruit diseases of avocado and how to manage them’ – Liz Dann (29 slides)

 


Key information

  • Anthracnose is a serious fungal disease in Australia that is second in importance only to Phytophthora root rot.
  • Anthracnose is widespread and causes fruit rots which only become evident when fruit ripens. For this reason, affected fruit are not seen by growers and packers but only become evident once fruit ripen after purchase. This results in a serious loss of consumer satisfaction and buying confidence.
  • The spores germinate on wet fruit and remain dormant and unseen until fruit ripens.
  • In Hass the problem is further masked by its thick, dark skin.
  • There is a high incidence of anthracnose in the warm summer rainfall regions of Queensland and New South Wales.
  • In Western Australia there is a different species of the pathogen adapted to cooler conditions, so anthracnose is also an issue in this region.
  • In the Tristate region anthracnose can be an issue when rain occurs and/or overhead misting is used.
  • For effective control, an integrated pre-harvest and postharvest approach is needed that includes the following practices:

◦ Where possible, choose a rootstock which has a proven history of producing more robust fruit e.g. Velvick.
◦ Maintain a protective layer of registered fungicide (mostly copper protectants) on the fruit from fruitset right through till harvest by applying foliar sprays every 14 to 28 days. The spray interval depends on weather conditions.
◦ Canopy management conducted to allow better light penetration and picker access will also improve ventilation and fungicide spray coverage.
◦ Practice orchard hygiene by removing sources of spore inoculum from the canopy including old fruit and, if possible, dead branches and twigs.
◦ Maintain good pest control and also take steps to reduce wind rub to fruit as any type of skin damage predisposes fruit to infection.
◦ Apply a balanced nutrition programme to promote the development of robust fruit. This entails having sufficient calcium available and not applying excess nitrogen.
◦ Do not harvest in the rain or at any other time when fruit is wet.
◦ Apply a registered postharvest fungicide to fruit within 24 hours of picking.
◦ Avoid delays between picking, packing and dispatching fruit, and maintain the cool chain from harvest onwards.

 

anthracnose - disease

Anthracnose causes severe internal damage to fruit. Damage is characterised by a flesh rot that expands hemispherically from the point of infection.