Minimising secondary damage will be the key task facing orchardists recovering from hail storms.
Queensland Department of Agriculture and Fisheries extension officer Simon Newett said it was important for all growers to be prepared for potential hail damage.
“Physical damage to trees lays them open to attack by fungi and insects that take advantage of the wounds, and the loss of canopy exposes the branches to severe sunburn damage,” Mr Newett said.
“In the case of insects, tree wounds release chemicals such as ethylene that appear to act as magnets to some opportunistic insects such as borers.”
For this reason, Mr Newett said it was a good idea to apply a fungicide and insecticide treatment.
“A registered avocado fungicide such as one of the coppers is suitable. The insects most likely to be attracted are borers of various types such as the auger beetle (Xylopsocus gibbicollis) and other ambrosia or pinhole borers.”
The advice from Department of Agriculture and Fisheries (QDAF) Mareeba’s Ian Newton and NSW Department of Primary Industries entomologists Craig Maddox and Ruth Huwer is to consider the registered insecticide chlorpyrifos, which is effective against beetles, or trichlorfon which will also be effective and treat spotting bug at the same time if you have fruit present.
“Once the borers are inside the tree it is too late so an application within a few days then a follow up perhaps a week or two later is suggested,” Mr Newett said.
“The advice is to try and avoid using pyrethroids at this early stage of the season for their potential to result in a build-up of other insects such as scale.
“An azoxystrobin fungicide could be used instead of copper but shouldn’t be applied at the same time as chlorpyrifos because of incompatibility.
“The other thing to take action on as soon as possible is sunburn protection. With branches exposed as a result of the loss of leaf cover some sort of sunblock such as white acrylic paint or a proprietary sunburn protection product should be applied to newly exposed branches especially on the northern and western aspects.”
Mr Newett said these products could often be applied in diluted form through orchard sprayers but multiple applications may be necessary to get enough protection. To speed up the canopy re-growth you may also want to apply some extra nitrogen.
“With the loss of crop it does present an opportunity carry out some canopy management, just remember to protect the newly exposed branches and trunks from sunburn before the fast approaching hot weather arrives,” he said.
More information
If you have any queries or want to discuss your particular situation, please contact Simon Newett on 07 5381 1326, 0400 565 784 or simon.newett@daf.qld.gov.au.
Acknowledgements
Thanks to Chris Searle, Ian Newton (QDAF entomologist at Mareeba) and the NSW DPI entomologists Craig Maddox and Ruth Huwer for their advice.
This article is based on one first published in the Spring 2018 Talking Avocados and has been reproduced for the 18 October 2019 Guacamole.
Australia can now export fresh avocados to Japan. On this page, you will find information on growing regions, seasonality and currently accredited packhouses.
Australia produces avocados all year. Currently, the Australian avocado industry can supply the Hass avocado to Japan from our growing regions in Western Australia, the Riverland of South Australia and Tasmania from August to March.
Australian avocados are exported via airfreight, ensuring Japanese consumers have access to fresh, high-quality Australian fruit. Australian avocados are mainly used in savoury dishes but can also be used in sweet dishes such as smoothies, ice creams and cakes.
Key contact: Trevor Bendotti
Phone number: +61 424 185 010
Email: admin@bendottiavocado.farm
Location: Pemberton, Western Australia, 6260, Australia Harvest/export season: July to March
This article appears in the Winter 2019 edition of Talking Avocados (Volume 30 No 2).
By Louisa Parkinson and Andrew Geering, Queensland Alliance for Agriculture and Food Innovation (QAAFI), The University of Queensland
The Australian avocado industry is relatively free from exotic pests and diseases, due to its geographic isolation and strong quarantine system. However, there are threats of exotic pest incursions that require novel diagnostic and surveillance tools to provide a capacity to respond to incursions.
This R&D update will summarise the biosecurity innovations that are being adopted in the Australian avocado industry, including the roll-out of a digital social networking tool (Checkpoint™) for pest surveillance; new molecular tests for detecting avocado scab fungus, Elsinoë perseae (syn. Sphaceloma perseae); an investigation of the fungal pathogens vectored by the Tea Shot Hole Borer (TSHB) Euwallacea fornicatus; and a new survey project investigating avocado sunblotch viroid (ASBVd).
Monitoring pest and disease threats with CheckpointTM
Checkpoint™ is an online social networking tool for on-farm recording of plant health data. The tool enables growers and crop protection consultants to instantly make an enquiry about a symptomatic tree and have direct access to diagnosticians in the laboratory. Checkpoint can be accessed via www.checkpoint.tools and can be used on smartphones and tablets for instant image uploading and record keeping. Pest and disease surveillance activities are recorded in real time with photographs, GPS coordinates, chat logs and steps in the diagnostic chain instantly saved to an online database via smart devices (Figure 1). Expertise can be drawn upon from anywhere in the country and scientists in a different capital city can be invited to contribute to the conversation to accelerate the diagnosis.
The aim of this project activity was to establish a network of researchers, extension officers, agronomists and growers who work in the industry that can report on current pest or disease issues in their region. A network, Australian Avocado Plant Health, comprising 39 industry stakeholders across all growing regions has been established through use of the Checkpoint tool. This project activity resulted in the adoption of private use of Checkpoint by two major avocado producing organisations in Australia for monitoring health of individual trees. Outcomes of the project include contributions and collaboration with software developers to improve the Checkpoint tool to suit industry needs; and building connections with avocado producers and agronomists through site visits for sample collection and providing plant pathology services.
Figure 1. Use of Checkpoint for recording branch dieback of avocado trees associated with ambrosia beetles.
qPCR detection of the high risk biosecurity threat, avocado scab fungus
The avocado scab fungus, Elsinoë perseae (syn. Sphaceloma perseae), is a high priority biosecurity threat for avocados in Australia. Avocado scab results in premature fruit drop and reduced fruit quality, which heavily impacts marketability and can restrict market access to pest-free countries. Scab symptoms begin with scattered corky, raised brown to purplish-brown lesions or ‘scabs’ which coalesce as the disease progresses, causing deep brown fissures covering the fruit surface.
Although scab symptoms are recognisable, confirmation of presence of E. perseae can be difficult, as the fungus is slow growing and can take up to two months for the fungal colony to grow on culture media in a lab. There is a need for a rapid, real-time molecular detection test for E. perseae to enable rapid responses to potential incursions.
A real-time quantitative Polymerase Chain Reaction (qPCR) diagnostic test has been developed for the rapid detection of E. perseae. PCR-based molecular tests amplifies nucleic acid sequences (such as DNA) of target pathogens by making millions of copies of the target sequence using primers which bind to the target sequences and an enzyme which catalyses the reaction to make the sequence copies. The E. perseae qPCR test incorporates fluorescent probes which specifically bind to target DNA sequences, are cleaved by the enzyme and release fluorescent molecules for detection in the qPCR machine. Detection can be visualised in real time (Figure 2) as more fluorescent molecules are released during the DNA sequence amplification process. The molecular test also checks for potential user error in sample loading by including an avocado endogenous gene which is simultaneously detected; samples should always give a positive result for avocado DNA regardless of pathogen presence in the plant sample.
This project activity has resulted in having the diagnostic capacity to quickly respond to potential E. perseae incursions if it were to happen to the Australian avocado industry.
Figure 2. Real time qPCR amplification of Elsinoë perseae. The red and blue lines represent amplification of two E. perseae cloned DNA samples (target pathogen DNA was artificially synthesized and cloned for use in validating the diagnostic).
Fusarium spp.
Fusarium dieback of avocado trees in Australia is vectored by ambrosia (scolytid) beetles in the Euwallacea fornicatus cryptic species complex (a group of closely related organisms). In Australia, the beetle species is known as the Tea Shot Hot Borer (TSHB), and it is found in South East Asia, Australasia and parts of the USA. The beetles carry symbiotic fungi in their mouthparts, bore into tree branches and deposit the fungus onto xylem tissue within the brood gallery for beetles and larvae to feed. Symptoms of ambrosia beetle related branch dieback include pin holes on branches with leaf wilt and localised branch death due to the vascular damage caused by the fungus.
During the last decade, there have been sporadic infestations of TSHB in avocado orchards in Australia and it appears to be the most severe on the Atherton Tableland in Far North Queensland. This research is investigating the phylogenetic diversity and pathogenicity of Fusarium species associated with branch dieback of avocado and other woody tree hosts in Australia.
The project activities to date include ambrosia beetle and branch dieback surveys across growing regions in Queensland, in Far North and Central Queensland and the Sunshine Coast region. Fungal isolates were obtained from symptomatic heartwood of borer-beetle affected branches of avocado trees and other hosts (including macadamia, mango, blueberry, Acacia, Cupaniopsis, Cyclophyllym and Alectryon), and from Euwallacea sp. beetle specimens collected from tree branches and traps. Fungal isolates were then identified and the fungal isolate collection contains now 142 isolates and the genera found associated with branch dieback of multiple tree hosts including Fusarium, Bionectria, Colletotrichum, Lasiodiplodia, Phomopsis, Nectria and Nigrospora. The preliminary work on the Fusarium genus has identified a possible new species of Fusaria associated with ambrosia beetles. Formal classification and descriptions of the new fungal species will be carried out in the next phase of the project, along with pathogenicity testing of selected fungal isolates on avocado and alternate hosts to identify the causal agents of beetle-mediated branch dieback.
Avocado sunblotch viroid – a new project
In 2018, a review of the status of avocado sunblotch viroid (ASBVd) in Australia was published and strategies for achieving pest-freedom status provided (Geering 2018). It was concluded that ASBVd is uncommon in Australia and is likely to be eradicated through continuation of disease management programs such as the Australian Avocado Nursery Accreditation Scheme (ANVAS). To date, the only records of ASBVd are from the Tristate region, northern NSW, south-eastern Queensland, and the Atherton Tableland. An ASBVd survey project (AV18007) has started, to provide updated data on the national distribution of the viroid, and will continue through to December 2021.
More information
If you would like to be a part of the Australian Avocado Plant Health network and/or trial the use of Checkpoint for your business, or if you suspect any of the biosecurity pest or diseases in your orchard or nursery, please email Louisa Parkinson (l.parkinson@uq.edu.au).
If you suspect you’ve found a new pest or disease, call the Exotic Plant Pest Hotline (Plant Health Australia) on 1800 084 881.
Further reading
Geering, A. D. (2018). A review of the status of Avocado sunblotch viroid in Australia. Australasian Plant Pathology, 47(6), 555-559. https://doi.org/10.1007/s13313-018-0592-6
Acknowledgement
This research is part of Avocado industry biosecurity capacity building (AV16010), funded by Hort Innovation using the avocado research and development levy and contributions from the Australian Government. This project is jointly supported by the Queensland Department of Agriculture and Fisheries (DAF) and the University of Queensland.
This article was prepared for the Winter 2019 Talking Avocados magazine.
The disease management project AV16007 started in November 2018 and concludes in May 2022. There are several activities being undertaken, across the major root, fruit and nursery diseases impacting avocado productivity in Australia. This article presents an overview of the project and update on experimental results obtained so far. More detailed research articles will be published in future issues of Talking Avocados.
Monitoring phosphonate residues in fruit
This activity is being conducted in collaboration with Graeme Thomas, GLT Horticulture, and several growers and agronomists across Australia who have kindly sent fruit samples for analyses. Hard green fruit was collected at commercial maturity from 40 blocks across Queensland and Western Australia. The ranges of phosphorous acid residues are presented in Table 1.
Table 1. Phosphorous acid residues in fruit harvested at commercial maturity in 2018
Region
Number of blocks
Minimum phos. acid (mg/kg fresh)
Maximum phos. acid (mg/kg fresh)
North QLD
12
15
96
Central and southeast QLD
13
3
93
Southwest WA
12
53
210
The maximum residue limit (MRL) for fruit sold within Australia is 500mg/kg, so none of the fruit tested came close to exceeding domestic MRL. Growers and exporters should be mindful of MRLs imposed by our current and potential overseas markets of all crop protectants used within the orchard, (see the updated MRL comparison table at in the Best Practice Resource Library, under the Export heading (www.avocado.org.au/bpr/). Results from the phosphonate residue testing have been communicated to participating growers and the Avocado Export Project Reference Group, and we are currently collecting Bundaberg/Childers fruit from the same blocks for the second year of testing. Information provided by growers on dates of phosphonate applications and concentrations in roots will be analysed with fruit residue data, providing important information on optimal delivery and timing of applications. We will also look for correlations with crop load (yield) and tree age.
Biofumigation for Phytophthora infested replant sites
A trial was initiated in northern NSW at a site where trees declining from Phytophthora root rot had recently been removed. Despite the high natural inoculum, we broadcast wheat grain colonised with Phytophthora cinnamomi to ensure consistent and even distribution of inoculum. A row (about 100m long) was cultivated and some plots sown with either Caliente 199TM or BQ Mulch, which are both Brassica cover crops commercialised specifically for their biofumigation effects, or left fallow. At flowering, the brassica crops were mown with several passes of the slasher and incorporated by rotary hoe. Chicken manure was incorporated into half of the fallow plots, and the other half left as untreated controls. One side of the row was covered in heavy duty black builders film, as shown in (Figure 1) for two weeks. Nursery trees (Hass on Reed) were planted two weeks after removal of the black plastic, with eight trees per treatment plot (four in each of the plastic covered and uncovered halves of the plots). Tree health and other growth parameters has been assessed at least monthly since planting. At six months after planting, the health of trees in covered plots was significantly better than those in uncovered plots (Figure 2), although there were no statistical effects amongst biofumigation treatments. We will continue to record tree health for the next couple of months.
Figure 1. Biofumigation trial site in Northern NSW. A length of black builders plastic covered half the row for two weeks after incorporation of the Brassica cover crops or chicken manure. Hass on Reed trees were planted two weeks after plastic was removed.Figure 2. Biofumigation trial 7 months after planting into a site heavily infested with Phytophthora cinnamomi. Trees planted into plots covered with plastic (left row) for 2 weeks after biofumigant incorporation are clearly healthier with fewer deaths than those planted into uncovered plots (right).
Field trials to assess effects of new chemicals and soil amendments on tree health, fruit yields and quality
This is a major component of the project. Two trials in south-west Western Australia and one trial in Central Queensland were established nearly 12 months ago in partnership with growers and agronomists. Treatments common to each site include woodchip + chicken manure + gypsum (as the recommended “best practice”), as well as Mineral Mulch (building board waste with high available Si and Ca, see Talking Avocados, Summer 2018) and anti-oomycete metalaxyl-M + unregistered product. There are additional microbial formulations, biochar and other mulches specific to each trial. Tree health assessments have been undertaken at each visit. Our first fruit harvest is coming up in July for the Childers trial, with WA later in the year. We will be assessing yields and packouts for each treatment, as well as levels of postharvest disease, anthracnose and stem end rot. We will sample leaves, fruit and soil for major nutrients and look for useful correlations as potential predictors of tree health and fruit quality. The treatment programs and assessments at all sites will continue until the conclusion of the project in May 2022.
Stem end rot, graft dieback (nursery), branch cankers and branch dieback
A range of fungi are associated with fruit stem end rot, graft dieback (in the nursery and sometimes after planting), branch cankers and branch dieback. These include Botryosphaeriaceae family (eg. Lasiodiplodia theobromae and Neofusicoccum parvum), Colletotrichum spp., Pestalotiopsis sp. and Diaporthe sp. While we frequently isolate these fungi onto agar media, we have little knowledge about which are the most pathogenic, ie. cause the most severe disease, or how they enter the fruit or branch. A PhD student commenced in April 2019 to investigate the fungi associated with the different symptoms in the major growing regions of Australia. So far more than 200 isolates have been collected from several orchards and nurseries, and these will be accurately identified by microscopy and molecular DNA sequencing. Any patterns showing which fungi are more prevalent with a particular symptom type or region will be determined. Modes of infection will be investigated, eg, whether they are carried as symptomless infections in nursery trees (as “endophytes” [1]), or enter via wounds or at flowering. A field trial in an unsprayed orchard in SE QLD is planned for later this year, where several treatments including fungicides and microbial/biological products will be sprayed at flowering and resulting fruit collected and assessed for development of stem end rot. This trial will also provide some information about new fungicides or products which may be safe to apply without burning sensitive flowers.
Figure 3. Soft, ripe Reed fruit with severe fungal disease through to the seed cavity.
Some preliminary lab and glasshouse work by an undergraduate student has shown that L. theobromae and N. parvum causing Stem End Rot and disease in fruit (Figure 3) are able to colonise the seed coat and seed (Figure 4). When these infected seed are planted (Figure 4b) the seedling stems are also infected by these fungi even though they may not show obvious symptoms. This can potentially cause graft failure in the nursery (Figure 5), or dieback from the graft after planting, as the infection may remain dormant in the graft until trees are stressed. It is extremely important that nurseries collect only clean fruit and extract the seed when the fruit is still hard prior to ripening (but must be physiologically mature). This removes the seed coat and reduces the risk of transfer of these endophytic fungal pathogens. This recommendation has been included in the recently-revised guidelines for ANVAS nurseries, and is available in the Nursery Industry Accreditation Scheme Australia (NIASA) Best Management Practice Guidelines, 7th Edition, updated 2018, Appendix 13 Avocado High Health Production. The guidelines are an excellent resource for any nursery, and can be purchased for $99 from http://nurseryproductionfms.com.au/niasa-accreditation/).
Figure 4a and 4b. Reed seed with discrete brown lesions caused by Botryosphaeria fungi.Figure 4a and 4b. Reed seed with discrete brown lesions caused by Botryosphaeria fungi.Figure 5. Graft dieback in the nursery can be caused by infection with the same fungi associated with fruit stem end rot.
Other diseases and activities
The project also includes field and glasshouse experiments evaluating Trichoderma and fungicide drench management options for Phellinus brown root rot and black root rot (see Talking AvocadosSpring 2017). There is also a significant industry support component including participation in grower field days and biosecurity advisory panels. We also interact directly with growers, agronomists and nursery operators and process samples to assist with diagnosing diseases or non-disease disorders, which may be impacting tree health or fruit quality. One of the more challenging of these has been the tree lodging problem (see Talking Avocados, Summer 2019 edition for more on that), which most likely arose from planting root-bound trees, rather than from a disease.
Acknowledgement
The Improving avocado orchard productivity through disease management (AV16007) project has been funded by Hort Innovation, using the avocado research and development levy and contributions from the Australian Government.
References
Dann and project collaborators (2018) Does silicon amendment benefit avocado tree health or fruit quality? Talking Avocados 28(4): 53-56.
Parkinson, E. Dann and R. Shivas (2017) Black root rot of avocado – what do we know and how can we manage it? Talking Avocados 28(3):35-39.
Dann, K. Bransgrove, S. Newett, G. Thomas and several growers (2019) Lodging of avocado trees, Talking Avocados 29(4):40-45.
[1] Endophytic fungi internally infect living plant tissues without causing any visible disease for at least part of their life cycle
This article was prepared for the Winter 2019 Talking Avocados magazine.
This article appears in the Winter 2019 edition of Talking Avocados (Volume 30 No 2).
By Amnon Haberman and Harley M. Smith, CSIRO Agriculture and Food
Avocado is a low yielding tree crop with average annual production levels equivalent to approximately 10t/ha, which is considerably lower than the theoretical value of 32.5t/ha, as estimated by Wolstenholme 1987. Low yields are attributed to the semi-domesticated nature of avocado1, due to unfavourable traits including vigorous shoot growth, excessive flowering, low fruit set and high fruit abscission2,3. In addition, avocado has a high propensity for alternate (biennial) bearing4. The predicted rise in global temperatures will likely enhance these traits and further reduce annual yields5. Together, the additive effects of these yield-associated traits limit production and present a major challenge for Australian orchard management for maximising yields and reducing seasonal variation.
Challenges in Australian avocado production
Yield associated traits are controlled by the interaction between the genetics, climate and management inputs6, as well as the age of the tree. To increase Hass yields, new management tools must be developed to reduce the impact of excessive vigour, low fruit set, high fruit abscission and biennial bearing. To achieve this, it is necessary to have a basic understanding of the physiology that drives these yield associated traits. Fruit abscission is a central component controlling fruit production and this process is poorly understood in avocado, as well as other fruit tree crops. Therefore, this article is focused on fruit drop, which is the major aim of the Hort Innovation funded project, AV16005.
Early fruit abscission
During the early fruit abscission event, a majority of fruitlets abscise within the first five weeks after fruit set7,8. The initial phase of the early fruit drop event is due to the abscission of unfertilised fruitlets7. The later phase of this fruit abscission event involves the abscission of fertilised fruitlets, typically between 6-10mm in size. Growers estimate that approximately 30-50% of the fertilised fruitlets drop during the early fruit abscission event.
Interaction between the spring flush and developing fruitlets
Due to the coincidence of vegetative and reproductive growth in the spring, it has been proposed that the early fruit abscission event is mediated in part by the growing spring flush, which competes with the developing fruitlet for photosynthates and nutrients (reviewed by Salazar-García et al. 2013). In support of this hypothesis, Salazar-Garcia and Lovatt (1998) reported that ‘functionally determinate’ inflorescences are three times more productive than indeterminate inflorescence shoots. Paclobutrazol and uniconazole are growth retardants that reduce stem elongation and leaf expansion via inhibition of gibberellin biosynthesis. As elongating stems have a high growth potential, an increase in yield by applications of paclobutrazol at flowering was associated with an augmentation in the number of fruits11,12. Applications of paclobutrazol also increased fruit size, which also contributes to higher yields13. However, other reports showed that applications of paclobutrazol, as well as uniconazole, at flowering did not increase yield14-16. In addition, studies showed that removal of the spring flush or applications of paclobutrazol increased fruit set; however, yield was not increased due to a heavy fruit drop during the summer in the treated trees17,18. The discrepancy of the effect of paclobutrazol and/or uniconazole on fruit drop and yield demonstrates the underlying complexity of the early fruit abscission event and mechanism(s) the mediates resource (carbohydrates) distribution to actively growing tissues in the tree.
Challenges for studying the early fruit abscission event
One of the major challenges for studying the early fruit abscission event is the low rate of fruit set followed by a high rate of fruitlet drop (reviewed by Salazar-García et al. 2013). The combined effect of these two traits severely reduces the ability to directly compare developmental profiles between retained and abscising fruitlets. This is extremely important, as studies in model plant systems, show that early fruit development is marked by massive changes in fruit physiology, including hormone signalling and gene expression19,20. Therefore, if retained and abscising fruits are not collected at the same developmental age, then it becomes extremely difficult to compare the physiological differences in order to identify the key factors that mediate abscission. Moreover, this hindrance also obstructs the ability to effectively study the interaction between the vegetative flushes and developing fruits. However, a basic understanding of the physiological basis of the summer fruit drop event will likely apply to the early fruit abscission event.
Summer fruit drop
The integration of genetic determinants, climatic events and management practices has impact on tree physiology and resource (carbohydrates) availability. As tree carbohydrate levels are essential for growth, the adjustment of crop load in response to resource availability is hypothesized to be a major factor that regulates the summer fruit drop event 3,21. Therefore, understanding how tree crop load is adjusted in response to resource availability and the physiological mechanism(s) that mediate fruit abscission may provide the knowledge required to develop new management tools to reduce fruit drop and increase production. Moreover, new management tools aimed at reducing the summer fruit drop will likely be effective for reducing the early fruit drop event.
Role of seed coat in fruit abscission
Experimental studies demonstrate that seed development is required for fruit retention and development7,8. The seed coat is the maternal component of the seed, which functions to provide the embryo with photosynthates and nutrients required for growth22. Moreover, the seed coat also synthesises plant growth regulators/hormones critical for regulating embryo development (reviewed by Bower and Cutting 1988; Robert et al. 2018). Interestingly, seed coat senescence is an observable characteristic associated with abscising fruits8,25 (Figure 1). Therefore, the seed coat function appears to be a critical tissue that determines the fate of a fruit, retained versus abscised.
Figure 1. Seed coat senescence is associated with fruit drop. Fruits firmly attached to the tree (A) display little seed coat senescence compared to fruits undergoing fruit abscission (B). Embryo, Em; seed coat, SC.
Model of fruit abscission
We have developed a model to explain fruit abscission in avocado. In this model, a subset of fruit in a tree undergoes abscission in response to a resource availability signal(s). As pointed out above, the physiology of the tree is speculated to be a key determinant of fruit drop (illustrated in Figure 2). In addition, competition for resources between fruits and with shoots also drive fruit drop. At this time, the nature of this resource availability signal(s) is unknown. The fruit abscission event is viewed as a multistep process in which the resource availability signal(s) mediate fruit growth cessation.
Figure 2. An illustration of resource competition based on an avocado branch with two developing fruits and a vegetative spring flush. Red arrows indicate conceptual interactions between growing vegetative and reproductive units of the shoot that are implicated in the regulation of fruit abscission.
Given that the seed coat plays a major role in fruit development and senescence of this tissue is associated with abscission, it is highly likely that seed coat mediates the cessation of fruit growth. After growth cessation, the seed coat undergoes senescence and the abscission zone is activated in the pedicel, which leads to the physical separation of the fruit from the tree. Therefore, the primary event of fruit abscission is fruit growth cessation, while the secondary event involves the process that mediates fruit drop. Based on this model, fruit abscission can only be reversed during fruit growth cessation. Once seed coat senescence is initiated, the cessation of fruit growth cannot be reversed. Therefore, in order to develop new tools to limit fruit abscission, an understanding of the physiological basis of fruit growth cessation is required.
The AV16005 Hort Innovation funded project
The primary aim of the AV16005 Hort Innovation funded project is to study the physiology of fruit growth cessation, as well as seed coat senescence and fruit abscission. However, we are lacking the capability to distinguish fruits fate to develop to maturity from fruits targeted for abscission, during early stages of fruit growth cessation. To overcome this problem, trials were performed to identify treatments that would induce a massive fruit drop event by limiting carbohydrate availability. Results from these trials showed that extensive removal of new vegetative growth promotes a massive fruit drop event. Using this approach, different fruit tissues, as well as pedicels and stems, were collected at regular time intervals from treated and control trees.
We are currently analysing the tissues using analytical and molecular methods to identify candidate hormones, metabolites, carbohydrates and genes that correlate with fruit growth cessation. This information will be integrated and used to construct the physiological and developmental pathways that mediate fruit growth cessation. Finally, these pathways will be incorporated into the model above, which will serve as a knowledge base for developing new management tools to limit fruit abscission.
We acknowledge and thank the contribution of Jasper Farms (WA), Delroy Orchards (WA), Chinoola Orchards (SA) and Thiel Orchards (SA) to the project and technical assistance from Jacinta Foley (Jasper Farms) and Declan McCauley (WA DPIRD).
Acknowledgement
The Maximising yield and reducing seasonal variation (AV16005) project has been funded by Hort Innovation, using the Avocado research and development levy and contributions from the Australian Government.
9. Salazar-García, S., Garner, L. C. & Lovatt, C. J. Reproductive biology. The avocado: botany production and uses. 2nd (Ed.). CABI, Oxfordshire, UK 118–167 (2013).
Avocados Australia CEO John Tyas is in India this week, as part of a Hort Innovation industry visit, arranged with the Australian Trade and Investment Commission, and he believes there are opportunities for the industry.
He says:
This market visit is providing Australian fruit industries with important in-country experiences. The Indian market has a lot of potential with a large population, and a high percentage of younger consumers looking for healthy options, including fresh fruit.
The 2019 Australian fruit market tour group, visiting Lots Wholesale Solution, in Uttar Pradesh.
The market in India is very diverse, from street carts to high-end department stores. It is also very price sensitive, but avocados are a bit of an exception at the moment. We have seen both Indian grown (green skin) and imported avocados (from Peru), on shelves this week.
Avocados are surprisingly prominent with upmarket stores displaying avocados at the front of the store, with very modern merchandising. I have seen 4kg trays of Peruvian Avocados for $50 in wholesale markets and $25 per kilogram at retail.
Everyone we have met with this week has confirmed that Australian avocados have a very good opportunity in India. We need to push even harder to get the Australian Government Department of Agriculture to negotiate market access as soon as possible. The opportunity is now.
The market program visits ranged from cash & carry businesses through to importers. The avocado, table grape, apple, pear, citrus and summerfruit industries were represented on the tour.
This article was prepared for the 26 July 2019 edition of the Guacamole.
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The bacterium Xylella fastidiosa is one of the most serious biosecurity threats to all of Australian agriculture, as it has one of the widest host ranges of any plant pathogen and causes economically-important diseases in many crop and ornamental plant species including grape, citrus, olive, coffee, oleander, peach, plum, almond, lucerne and avocado (for a regularly updated database of host species, see www.efsa.europa.eu/en/microstrategy/xylella). This bacterium inhabits the xylem of the plant and causes blockages that prevent water and mineral transport. It is speculated that almost any xylem-feeding insect could transmit the bacterium, hence there is a strong likelihood that an endemic insect species could act as a vector, in the event that only the bacterium was introduced into Australia. Known vectors elsewhere in the world include sharpshooter leafhoppers (Hemiptera: Cicadellidae: Cicadellinae) and spittlebugs (Hemiptera: Cercopoidea).
Xylella fastidiosa is a genetically diverse bacterium, with five recognised subspecies (subsp. fastidiosa, multiplex, pauca, sandyi and morus) and additional strains within each subspecies. The strong consensus is that X. fastidiosa evolved in the Americas but with different geographical origins for each subspecies; subsp. multiplex is believed to have originated in North America, subsp. fastidiosa in Central America and subsp. pauca in South America. Understanding genetic diversity is important, as the different subspecies and strains have different host ranges, and are transmitted at varying efficiencies by the different insect species. How different bacterial genotypes, insect vectors and host plant species interact to cause disease epidemics is poorly understood. In the USA, X. fastidiosa is transmitted by native sharpshooters but the introduction of the glassy-winged sharpshooter (Homalodisca vitripennis) to California in the late 1990s led to a dramatic increase in the incidence of disease in grapevines.
For much of time, X. fastidiosa remained confined to the New World. However, X. fastidiosa was detected for the first time in Europe in 2013 as part of studies to determine the cause of a disease that was devastating ancient olive groves in southern Italy. Following this incursion, delimitation surveys were undertaken and the pathogen was also found in France and Spain in more than 30 host species, including Oleander, cherry, almond and many endemic species in the Mediterranean flora. Genetic studies suggested a single, recent introduction of the pathogen into southern Italy from Costa Rica. This pathogen incursion would have been facilitated by humans, as the insect vectors would not have had the capacity to cross the Atlantic Ocean by their own means. Trade in ornamental plants is thought to be the pathway by which the bacterium entered Europe.
Given its very broad host range, it is not surprising that X. fastidiosa also infects avocado. The first published report of disease in avocado trees caused by this pathogen was from Costa Rica in 2007 (Montero-Astúa et al., 2007). In this study, infected trees were found at two distinct geographical locations, in mountains to the north and south of the Central Valley (Alajuela and San José provinces). Disease symptoms included:
chlorotic mottling, marginal scorch and deformation of the leaves (wavy edge; sometimes shorter on one edge, giving a crescent shape)
defoliation
shortening of internodes;
branch dieback;
an uneven distribution of symptoms across the tree, with some branches appearing healthy.
Some of these symptoms are illustrated in the accompanying photographs (Figures 1-3).
Figure 1. Chlorotic mottling caused by Xylella fastidiosa (photograph courtesy of Mauricio Montero Astúa)
Little is known about the nature of epidemics of X. fastidiosa in avocado in Costa Rica. It is likely that the pathogen is more widespread within avocado orchards in Costa Rica and in neighbouring countries of Central America. Knowledge of pathogen strain diversity and insect vectors in avocado orchards is also non-existent, which is critical information for understanding disease epidemiology. Finally, problems were encountered with obtaining pure cultures of X. fastidiosa from avocado, possibly due to the mucilaginous sap that was released when the leaves were sampled. Clearly, more research is needed considering the seriousness of this disease.
The only other report of X. fastidiosa infecting avocado is from California (California Minor Crops Council, https://ipmdata.ipmcenters.org/documents/pmsps/CAavocado.pdf). While avocado is not a preferred host of the glassy-winged sharpshooter, this insect pest will infest avocado trees when other suitable hosts are not available for feeding or when populations of the sharpshooter are very high, particularly when avocado and orange are grown in proximity to each other. X. fastidiosa has been detected in avocado but these infections were not associated with any symptoms. These observations contrast with those from Costa Rica, most likely due to genetic differences between the bacterial populations in the two regions. In California, the glassy-winged sharpshooter alone causes economic losses to the avocado farmers as it feeds on the fruit stalk and deposits excrement over the fruit, reducing its marketability.
It is no wonder that X. fastidiosa is rated the most important biosecurity threat to Australian horticulture by the Department of Agriculture and Water Resources. If symptoms similar to those in Figures 1, 2 and 3 are noticed by anyone in Australia, it is very important to notify biosecurity agencies as soon as possible (please call the Exotic Plant Pest Hotline on 1800 084 881) or contact us by email (L.Parkinson@uq.edu.au). If X. fastidiosa was to establish in Australia, the viability of many horticultural industries would be at risk.
Figure 2. Leaf deformation caused by Xylella fastidiosa (photograph courtesy of Mauricio Montero Astúa)Figure 3. Shortening of internodes caused by Xylella fastidiosa (photograph courtesy of Mauricio Montero Astúa)
Further reading
Montero-Astúa M, Saborío-R G, Chacón-Díaz C, Garita L, Villalobos W, Moreira L, Hartung JS, Rivera C (2007) First report of Xylella fastidiosa in avocado in Costa Rica. Plant Disease 92 (1):175-175. https://apsjournals.apsnet.org/doi/10.1094/PDIS-92-1-0175C
We thank Dr Mauricio Montero Astúa for useful discussions and kindly providing photographs of disease symptoms. This project has been funded by Hort Innovation, using the avocado research and development levy and contributions from the Australian Government.
This article was prepared by the authors for the Autumn 2019 edition of Talking Avocados (number 30, volume 1).
Farm and packhouse managers must complete the application form and submit the completed form to Avocados Australia by 7 June 2019. Late submissions will not be accepted.
Please download the documentation directly from the Department of Agriculture and Water Resources by clicking here.
Key points from the DAWR Industry Advice Notice
Only fresh avocado (Persea americana) fruit of the Hass cultivar are permitted to be exported to Japan.
Japan Ministry of Agriculture, Forestry and Fisheries (MAFF) protocol conditions apply to fresh Hass avocado sourced from officially recognised areas free from Queensland fruit fly (Qfly) in Western Australia and Riverland (South Australia).
Avocados Australia will forward accreditation applications to the department.
The department may conduct audits prior to confirming accreditation.
This article was prepared for the 17 May 2019 edition of the Guacamole.
Avocados Australia has managed the Kangaroo Label for use on Australian grown avocados since 2011, and there’s recently been a significant update. Avocados Australia CEO John Tyas said the major change was the introduction of databar by GS1 and the need for packhouses and growers to manage their own databar requirements.
“Avocados Australia is proud to continue with and support the use of the uniquely identifiable Kangaroo Label for use on Australian grown Avocados,” Mr Tyas said.
“With the introduction of the new databar requirement (GS1) to replace the previous barcodes managed by Avocados Australia, we have worked with our registered printers to update and refresh the label to accommodate the databar.
“All packhouses using barcode labels will be required to apply to GS1 for their own databar, which can then be easily inserted into the Kangaroo Label of your choice (options pictured).”
The industry now has a variety of new designs for its Kangaroo Labels. Images courtesy of Warehouse Design & Packaging.
Packhouses will still need to apply to Avocados Australia for a Packhouse Registration Number (PRN), which will authorise them to use the Kangaroo Label. Once a PRN application is approved by Avocados Australia, the packhouse will receive a unique PRN and the Registered Label Suppliers will be informed of the new PRN.
Packhouses may then contact any of the Registered Label Suppliers directly to order Kangaroo Labels. Please contact one of our registered printers to discuss your labelling requirements.
