Improving avocado orchard productivity through disease management (AV16007)

The project continues to provide highly relevant information on the major diseases affecting avocado productivity and their effective management.  Over the last couple of years, investigations of the causes and management of panicle dieback have delivered some useful information and indicate a strong fungal association. Final testing in the field with several fungal isolates is currently underway to demonstrate which genera may cause more severe panicle dieback if conditions are conducive. Two seasons of field trials in an orchard with severe panicle dieback have shown that fungicide applications at flowering and early fruit set do not cause phytotoxicity to flowers, and some fungicide treatments result in increased yields at commercial harvest.

Industry support activities have become a major component to the project. In addition to those reported previously, further dedicated disease management workshops were held in Port Macquarie, NSW (June 2022) and Walkamin, QLD (August 2022). The presentations focused on fruit quality (anthracnose and stem end rot), and Phytophthora root rot, and included some basic information about disease cycles, integrated disease management, and research project highlights.

Project staff also participated other industry extension events, as part of AV17005, including the Avoskills workshop at Port Macquarie, and Industry field days at Wauchope and Alstonville, NSW. The workshops were well attended, with good informal interaction and positive feedback.

With oversupply of fruit in domestic markets, more fruit will be forced into export markets and/or into longer supply chains. This means the quality of fruit being harvested is more critical than ever.  Fruit must be protected throughout their orchard life, from infections by fungi causing postharvest diseases such as anthracnose and stem end rot. Correct and timely fungicide applications specific to each region, and other management strategies to reduce field infections, have been highlighted in the workshops.

The issue of compliance with strict maximum residue limits (MRLs) for pesticides specific for each export destination has been raised and discussed amongst stakeholders, and general awareness within industry has been heightened over the last 12 months.

Project progress continues…

Three soil amendment trials are underway with fruit yields, pack outs and postharvest disease severity data obtained for three commercial harvests at each site. Soil drench applications with oomycete specific fungicides have improved tree health in the WA trials, leading to increased yields in 2021, at one site. Higher yield was associated with increased premium pack out, and correlations between fruit pulp nutrients and stem-end rot severity were observed.

Trials to evaluate fungicide application in mitigating panicle dieback were undertaken. Results indicated if applications were made at mid flowering and early fruit set, fungicide applications may be beneficial to fruit set and thus yield. One of the fungicides is registered by Nufarm in other crops, and the company have indicated that they are progressing with larger-scale field trials in avocado, with a view to registration, based on two years of promising results from the trial. The final harvest from the trial is due in late-May 2022.

Two trials evaluating soil drench fungicides for Verticillium wilt management have been initiated, with the first treatments applied in Spring 2021. Further applications in autumn and spring 2022, and monitoring for symptoms, will be done to determine the efficacy of treatments.

Laboratory work has commenced on screening Colletotrichum spp. isolates for potential resistance to the QoI (strobilurin) group of fungicides. Many Colletotrichum isolates (which causes anthracnose and stem-end rot of fruit), are being screened to adapt integrated management practices and recommendations

Industry support activities were implemented including two dedicated industry workshops on avocado disease management. These focused on fruit quality (anthracnose and stem-end rot), and Phytophthora root rot, and included information about disease cycles and integrated disease management.

Three soil amendment trials have been under evaluation since mid-2018, with fruit yields, packouts and postharvest disease severity data obtained across three commercial harvests at each site. Results showed none of the treatments had an appreciable effect on tree health at the Childers site in Queensland, however a soil conditioner treatment had the greatest cumulative yield. Trees in both the Western Australian trials were less healthy than at the Childers site, with those treated with fungicide drenches twice a year visibly healthier than trees in all other treatments. A trend was apparent for greater yields from mulched trees compared with unmulched.

Preliminary studies on panicle dieback showed that heavy pruning may help restore vegetative health of severely affected trees, with this season’s flowering and fruit set not exhibiting panicle dieback, in contrast to neighbouring trees. Therefore, pruning may be a realistic management strategy for affected trees or blocks. Fungicide applications at flowering did not prevent dieback, and while there was some treatment effect on yields, the differences were not statistically significant. Postharvest disease in fruit was significantly reduced by some fungicides that are not currently registered for avocado, compared to industry standard fungicide regime or untreated controls.

Industry support activities were a major component of project activities, including presentations at field days and workshops (in-person and web-based), and extension articles focused on growers and agronomists. Responding to numerous grower enquiries and diagnostic work undertaken on samples collected by project team or sent to us by growers or nurseries was also very important. Correct diagnosis of a disease or other problem is required for optimal (and cost-effective) management.

ACT NOW

Watch a recording of QAAFI Seminar Investigating threats to Our Green Gold: blight and dieback of avocado held on 17 August 2021

Read the team’s project update reported in Talking Avocados on page 49

The researchers report progress in the following areas:

Phytophthora root rot

Monitoring fruit from commercial blocks across Australia for phosphite residues is complete, demonstrating that higher residues are associated with increased applications of phosphonic acid.

Field trials with mulches, soil amendments and new chemistry are progressing. Data on fruit, leaf and pulp nutrient analyses and postharvest disease severity has been collected.

The Brassica biofumigation trial was completed, with inconclusive results into the effects of planting biofumigant species prior to replanting with avocado. However, the trial demonstrated that covering replant sites with black builders’ plastic for three weeks significantly improved the establishment of new trees where there was high Phytophthora cinnamomi infestation.

Phellinus brown root rot

A trial at Maleny, south east Queensland, was established with Trichoderma sprays applied in summer 2018/19 and 2019/20. This does not appear to be a promising treatment, as trees with existing infection have died.

Nectriaceous black root rot

Glasshouse trials confirmed that Mariannea spp. are not pathogenic on avocado. Some preliminary trials were completed, with further investigation required. The assessment of fungicides with post-infection efficacy should be completed in 2020.

Stem end rot

The testing of several isolates for their relative ability to cause stem end rot on fruit will be undertaken in the second half of 2020.

Branch and Graft dieback

Glasshouse trials demonstrated that some species (eg. Neofusicoccum parvum) cause more severe dieback than others, with a preliminary experiment identifying pruning wound treatments that reduce dieback severity. This experiment will be repeated, and the option to evaluate under field conditions will be reviewed.

Fruit quality

Fruit quality is a component of the field trials mentioned above. Several isolates of Colletotrichum, Neofusicoccum, Lasiodiplodia and others were collected from fruit, branch dieback, cankers and other symptoms, from most growing regions. These isolates have been stored and will be analysed in coming months.

ACT NOW

Read about Improving avocado fruit quality: evaluation of post-harvest treatments on page 58 of Talking Avocados, Autumn 2020 edition.

The researchers report progress in the following areas…

Phosphonate treatments and residue monitoring

Levels of phosphonate residue in fruit have been measured using fruit from 40 blocks (35 Hass and 5 Shepard) across Australia. None of the fruit exceeded the temporary maximum residue limit for fruit marketed in Australia. Most samples had residues between 20 and 100 mg/kg, with three samples having less than 20 mg and five blocks with over 150 mg.

The data indicates that for some blocks, applications of phosphonate may be occurring too early. This can result in more phosphonate accumulation in fruit and less in roots than is desirable. It also suggests that summer applications may contribute to residues when fruit are in the strong development phase.

Pre-plant treatment trials

Initial results are available from the field trials testing pre-plant biofumigants (Caliente, BQ Mulch, plus chicken manure) in a commercial avocado orchard with high natural Phytophthora cinnamomi infestation. Plots were replanted with Hass on Reed nursery stock four weeks after incorporation and some plots were covered in black plastic.  Monthly tree health assessments showed no significant differences between biofumigants, but trees in the plots covered with black plastic were significantly healthier than those in the uncovered plots.  Soil samples collected immediately prior to replanting showed covered plots had significantly higher levels of many nutrients including nitrate, ammonium, manganese, sulphur, chloride, magnesium and potassium.

Project team members continue to respond to enquiries from growers, orchard managers, nursery operators, agronomists and extension specialists. Verticillium wilt was a significant problem for many growers in WA in summer 2019 so the researchers investigated. Verticillium dahliae was confirmed as the causal pathogen, and researchers provided advice for growers.

Testing P. cinnamomi for resistance to phosphite

The team at Murdoch University is working to confirm that some P cinnamomi isolates are becoming resistant to phosphite. Their trial involving 600 trees exposed to variants of the disease will yield results in a few months.

Other work

The team is developing a new way to determine phosphite concentrations in plant tissues, and is carrying out work to find out the concentrations of phosphite required in roots to protect them from infection and colonisation by P. cinnamomi.

650 avocado seeds (cv Reed) were sown in April 2019 to provide plants for future aspects of the research.

ACT NOW 

• Read about Verticillium wilt in avocado, which has been a problem in Western Australia this summer in this article on the Avocados Australia website.
  
  
• Lodging of avocado trees is the subject of an article from the team on page 40 of the summer edition of Talking Avocados.

As previously reported, there are a number of different activities and experiments currently being undertaken by this investment. The project team have provided the below update on their work…

Several different trials underway

A significant achievement to date has been the establishment of three field trials for assessing different mulches, soil amendments and other treatments on avocado tree health, fruit yields and quality. Two trials have been initiated in south west Western Australia and one trial at Childers, Queensland. All trees were rated for canopy health, and soil and leaf samples were taken for analyses to provide the start of treatment reference point for key physio-chemical and nutritional properties. The grower collaborators have had significant input into the selection of treatments for their orchards.

The treatments will be conducted over the next three cropping seasons. Effects of the treatments on soil properties, including microbial activity, tree canopy health and nutritional status, fruit yield and postharvest quality will be assessed annually.

Pre-plant treatment trials

A field trial in northern New South Wales is investigating the efficacy of brassica biofumigant crops or chicken manure as pre-plant treatments, at a site with a high natural infestation with Phytophthora cinnamomi. The brassica crop was macerated and thoroughly incorporated into the top 10cm of soil. Half the plots were covered in plastic. Hass trees grafted onto susceptible rootstock Reed were recently planted, and will be monitored over the coming months for establishment success and canopy health.

Phosphonate treatments and residue monitoring

Phosphonate injections or foliar sprays are a key component in orchard management for Phytophthora root rot. The application timing is critical to ensure maximum translocation to the roots, and minimal translocation to the developing fruit.  Monitoring concentrations in roots has been commonplace for several years, however measuring residues in fruit is not routinely undertaken. Agronomists, growers and the project leader have sampled fruit at commercial maturity from over 40 blocks from north Queensland around to south west Western Australia during 2018. Samples have been processed for dry matter determination and analysed by the commercial lab for phosphorous acid residues. Collaborators have provided corresponding results from root testing, and data analyses will be conducted over coming months.

Other work

The project team has also been conducting surveys for fungi involved in stem end rot in fruit, graft and branch dieback, to better understand and tackle the pathogens. Field trials into Phellinus brown root rot management have also begun.

Meanwhile, in the winter of 2018 a significant issue emerged for many growers in Western Australia, with trees snapping off at ground level and blowing over. The project team are involved in ongoing efforts to identify the likely cause or causes of the problem, and produced this basic tree blow-over handout with pictures and descriptions of symptoms, what was known at the time, and preliminary results from lab testings.

In its initial months, this investment has been working to establish a range of trial work, including a field investigation looking at the use of pre-plant biofumigation to tackle disease. For this work, an orchard has been selected where the trees were in severe decline due to Phytophthora root rot, and biofumigants of the brassica species are to be studied. The researchers note that brassica biofumigants are quick growing and frequently used in the vegetable industry to break nematode and soilborne disease cycles, and add organic matter to soil. In late May, sowing with two biofumigants took place (Caliente and BQ Mulch), after the trees were cleared from the trial area. Avocado seedlings or grafted plants are set to be sown into the plot, and will be assessed for mortality or other symptoms of Phytophthora decline in the ensuing months.

Planning is also underway for larger field trials to be conducted over the project’s course, assessing different amendments or treatments on tree health, yield and fruit quality. At the time of writing, it was anticipated that the first treatment applications to selected sites would likely occur during July and August this year.

Meanwhile, glasshouse trials have begun, looking at the suppression of Nectriaceae fungi post-infection using a range of fungicides. These trials will assess effects on plant and root growth and root necrosis after inoculation with Calonectria ilicicola and Dactylonectria macrodidyma, and subsequent fungicide drench.

Finally, in relation to phosphite treatments for Phytophthora, the researchers have been looking at fruit at commercial maturity for phosphorous acid residues – with a number of growers submitting fruit for assessment. Initial results on residues are to be reported back to participating growers, and more widely to industry, so that a clear picture of optimal time for application of phosphonate sprays and injections can be determined.

In the Murdoch University component of the work, sampling is underway to look at phosphite tolerance in Phytophthora cinnamomi and more.

ACT NOW  

• Growers interested in learning more about the work, or taking part in trials or analysis, can contact University of Queensland project leader Liz Dann at e.dann@uq.edu.au
  
  
• Read more about the project work in this article
  
  
• In case you missed it, check out the article giving an overview of this work in the autumn 2018 edition of Talking Avocados