Optimizing phosphonate use for
Phytophthora root rot management in
Shepard avocados in North Queensland
Matthew Weinert
The Department of Agriculture, Fisheries and
Forestry, Qld
Project Number: AV11011
AV11011
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The research contained in this report was funded by
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Atherton Tablelands Avocado Growers Association Inc..
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AV11011 (30/09/13)
HAL FINAL REPORT
Optimising Phosphonate Use for Phytophthora Root Rot
Management in Shepard Avocados
in North Queensland
Matthew P. Weinert and Geoffrey R. Dickinson
Agri-Science Queensland
Department of Agriculture, Fisheries and Forestry (DAFF)
Queensland
HAL Project Number: AV11011
Project Leader: Matthew P. Weinert
Senior Extension Horticulturist
AgriScience Queensland, a service of the
Department of Agriculture, Fisheries and Forestry
PO Box 1054
MAREEBA QLD 4880
Phone: (07) 4048 4651
Email: matthew.weinert@daff.qld.gov.au
Key personnel: Dr Geoffrey R. Dickinson
Senior Horticulturist AgriScience Queensland, a service of the
Department of Agriculture, Fisheries and Forestry
PO Box 1054
MAREEBA QLD 4880
Phone: (07) 4048 4762
Email: geoff.dickinson@daff.qld.gov.au
Purpose:
The purpose of this report is to describe the activities and outcomes associated with the project
‘Optimising phosphonate use in Shepard avocados in north Queensland’.
Funding:
This project has been funded by Horticulture Australia Ltd (HAL) using voluntary contributions from
the Atherton Tableland Avocado Grower’s Association (ATAGA) and matched funds from the
Australian Government. The Queensland Government has also co-funded the project through the
Queensland Department of Agriculture Fisheries and Forestry.
30 September 2013
Disclaimer:
Any recommendations contained in this publication do not necessarily represent current HAL policy.
No person should act on the basis of the contents of this publication, whether as to matters of fact or
opinion or other content, without first obtaining specific, independent professional advice in respect
of the matters set out in this publication.
mailto:matthew.weinert@daff.qld.gov.au
- 1 -
CONTENTS
MEDIA SUMMARY .............................................................................................................................. 2
TECHNICAL SUMMARY .................................................................................................................... 3
INTRODUCTION .................................................................................................................................. 4
MATERIALS AND METHODS ............................................................................................................ 6
Trial design ......................................................................................................................................... 6
Measurements ..................................................................................................................................... 6
RESULTS ............................................................................................................................................... 8
Phosphonate levels ............................................................................................................................. 8
Phosphonate injection + Foliar Spray treatments ........................................................................... 8
Phosphonate injection only .......................................................................................................... 10
Shoot phenology ............................................................................................................................... 12
Flowering phenology ........................................................................................................................ 12
Root phenology ................................................................................................................................ 12
DISCUSSION ....................................................................................................................................... 14
Phosphonate levels ........................................................................................................................... 14
Phosphonate sprays ...................................................................................................................... 14
Phosphonate injection................................................................................................................... 14
Tree phenology ................................................................................................................................. 15
Root flushing ................................................................................................................................ 15
Shoot flushing .............................................................................................................................. 15
Flowering ..................................................................................................................................... 16
Extension outputs ............................................................................................................................. 17
CONCLUSIONS................................................................................................................................... 18
ACKNOWLEDGEMENTS .................................................................................................................. 20
BIBLIOGRAPHY ................................................................................................................................. 21
APPENDIX 1. North Queensland Shepard Crop Calendar ................................................................. 22
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MEDIA SUMMARY
Phytophthora root rot (PRR) is the most significant threat to Shepard avocado orchards in north
Queensland and needs to be managed carefully via cultural and chemical management techniques to
prevent widespread tree decline or deaths. It is important to note that the disease can not be controlled,
but must be actively managed.
The chemical potassium phosphonate, commonly called phosphorous acid or just phosphonate, is
recommended to manage PRR. Phosphonate moves inside the tree and after application, travels to the
most actively growing part of the tree. To effectively manage PRR phosphonate must reach the roots.
Timing applications to when roots are actively growing is of utmost importance. Monitoring
phosphonate in roots to ensure adequate levels are maintained is an important part of any program
The current application method, using tree injections is seen as a costly and labour intensive practice.
Due to the complexity and cost of injecting, some growers have moved to applying potassium
phosphonate treatments only once during the year, either through injections or foliar applications.
Maintaining root phosphonate levels through foliar sprays is more difficult to achieve than with
injections. Preliminary root monitoring data obtained from growers using foliar sprays prior to the
project showed that during the critical summer period root phosphonate levels were below that
required to manage the PRR.
Root monitoring for phosphonate levels was conducted on five Shepard avocado orchards in north
Queensland in 2012/13. In all five orchards trees continued to receive the farmer’s standard
phosphonate trunk injection program (either 1 or 2 injections per annum) over the duration of the trial
period. Three orchards were selected to receive the additional foliar phosphonate treatments.
Monthly root samples confirmed that foliar phosphonate applications were a highly effective way of
increasing avocado root phosphonate levels in healthy trees, when applied at/or shortly before the
periods of peak root flushing. As timing of application is influenced by tree growth cycles; root,
flower and shoot flushes were also monitored to determine their effects on phosphonate movement to
and accumulation in the roots
Two key time periods to apply phosphonate, based on shoot and root phenology and measured root
phosphonate levels, were identified. These periods were from March to June and mid November to
early December. Treatment by either application method (injections or foliar sprays) once a year
however, may not provide adequate phosphonate levels in the roots to provide protection for a full 12
months.
The key components to maintaining root phosphonate above recommended levels in Shepard
avocados within an integrated PRR program are: timing of phosphonate application based on root
growth flushes, monitoring root levels during critical times and applying the correct amount and
concentration, either as a high volume spray or through injections. Managing PRR using cultural,
including mulching and optimising nutrition and irrigation will also help optimise phosphonate
applications.
This project has been funded by Horticulture Australia Ltd (HAL) using voluntary contributions from
the Atherton Tableland Avocado Grower’s Association (ATAGA) and matched funds from the
Australian Government. DAFF Queensland provided in-kind support.
.
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TECHNICAL SUMMARY
Phytophthora root rot (PRR) of avocado (Persea americana), caused by the pathogen Phytophthora
cinnamomi remains the major threat to avocado production in Australia. This is despite 30 years of
research, development and extension into control and management. The pathogen attacks the fine
feeder roots of avocado trees and impacts on productivity by lowering yields, reducing fruit size,
quality and shelf life. Severe infection can kill trees. PRR is a significant problem in the seasonally
wet monsoonal climate of north Queensland, where many orchards are also grown on poor soils, low
in organic matter, with poor or impeded drainage.
Chemical control of PRR of avocados is achieved through a combination of cultural methods and the
use of the chemical potassium phosphonate which is applied to the tree as a trunk injection or foliar
sprays. Due to the perceived complexity and cost of injecting, some growers have moved to applying
potassium phosphonate treatments only once during the year either through injections or foliar
applications. Maintaining optimum phosphonate levels in avocado roots via foliar application can be
difficult to achieve as phosphonate is highly mobile and moves to the most actively growing plant
parts.
This study was conducted from May 2012 to May 2013, across five Shepard avocado orchards in far
north Queensland, on sites representing the range of local growing environments. In all five orchards
trees continued to receive the farmer’s standard phosphonate trunk injection program (either 1 or 2
injections per annum) over the duration of the trial period. Three orchards were selected to receive
the additional foliar phosphonate treatments. Monthly tree phenology measurements were conducted
and root samples collected from treatments in each of the 5 orchards to determine seasonal variation
in root phosphonate levels over the growing season and its relationship to tree phenology.
Two key time periods to apply phosphonate, based on both shoot and root phenology and measured
root phosphonate levels, were identified. These periods were from March to June and mid November
to early December.
Root phosphonate levels can be easily increased in healthy trees with phosphonate spray or injection
applications, when roots are actively flushing. This suggests that there is potential for management of
orchard root phosphonate levels via an integrated injection/foliar spray system or potentially via foliar
phosphonate applications alone. Treatment by either application method (injections or foliar sprays)
once a year however, may not provide adequate phosphonate levels in the roots to provide protection
for a full 12 months.
Study outcomes have improved understanding of Shepard tree phenology on the Atherton Tablelands
and the relationship to the uptake and movement of applied phosphonate (after injection or spray
application) in a range of management systems. A new ‘Crop Calender’ guide for Shepard avocados
on the Atherton Tablelands, accessible through the Avocados Australia Best Practice Resource
website, has been produced which will be a useful tool to guide farmers management practices.
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INTRODUCTION
Phytophthora root rot (PRR) of avocados (Phytophthora cinnamomi) is the most destructive and
important disease of avocados (Persea americana) worldwide and is the major limiting factor to
avocado production in Australia. The pathogen destroys the fine feeder roots of the avocado tree,
leading to water stress, nutrient deficiencies and increased salt burn as roots are unable to control salt
uptake. This in turn lowers yields, reduces fruit size, quality and shelf life, increases sunburn and the
number of reject fruit at packing. Severe infection can kill trees of all ages (Pegg et al. 2002). The
heavy monsoonal rains, warm soil conditions and in the main, poor soil types low in organic matter,
make conditions in the north Queensland (NQ) production areas on Atherton Tablelands and around
Mareeba-Dimbulah particularly favourable for PRR.
It is important to note that the disease can not be controlled, but must be actively managed. An
integrated program is currently recommended for Phytophthora root rot management in avocados
using a combination of cultural methods and chemical management strategies. This program is known
as the ‘Pegg Wheel’ (Figure 1). Potassium phosphonate, commonly called phosphorous acid or just
phosphonate, is the principal chemical component used in this management strategy.
Figure1. The Pegg Wheel.
Phosphonate is a systemic fungicide that moves in both the phloem and xylem of the tree (up and
down). Phosphonate can be applied as either trunk injections or foliar sprays. Following trunk
injections, phosphonate moves first to the leaves in the transpiration stream of the xylem and then via
the phloem to the site of the most actively growing part of the tree. Following foliar applications
phosphonate moves from the leaves via the phloem to the most actively growing part of the tree.
A root phosphonate level of 25 µg/g has been accepted as the minimum required level to manage
PRR, however recently a minimum root level of 40 µg/g has been proposed (Dann, 2011). Industry
recommendations for Hass variety trees are to conduct phosphonate injections to at-risk or infected
trees twice a year, to coincide with spring and autumn root flushes (usually March-May and
November). Optimum application timing requires an understanding of tree phenology, which varies
between avocado varieties and is influenced by local environmental conditions. Monitoring levels of
phosphonate in roots to ensure optimum levels are maintained is an important part of any program.
With reduced sensitivity reported from Phytophthora isolates from avocado orchards treated with
phosphonate in Australia and South Africa (Dobrowolski et al. 2008; Duvenhage 1994), it is
important that phosphonate application is optimized to ensure the continued future of this important
chemical in Australian avocado production systems.
Growers conducting avocado root analysis sampling in north Queensland have increased seven fold
following two workshops on Phytophthora management in north Queensland in 2009/10 as part of the
HAL funded ‘AV06003 – Study groups to Grow Globally Competitive Avocados’ project. Root
monitoring data in other Australian production areas have suggested the original twice/year
- 5 -
application recommendation could be replaced with an injection treatment once a year, after the
summer flush maturity (Graeme Thomas pers. comm. 2011). This practice of applying phosphonate
at a single time, either as injections or a foliar spray is also starting to become common practice in the
NQ growing region. Due to the complexity and cost of injecting, some growers have also expressed
interest to switch from injections to the practice of foliar application of phosphonate. Maintaining root
phosphonate levels through foliar sprays however is more difficult to achieve.
Preliminary root level data, primarily from Shepard orchards, from the north Queensland production
areas showed that with foliar applications post harvest, but before pruning, root levels greater than
150µg/g can be achieved. Follow up analyses prior to application the following year however found
levels well below the required 25-40 µg/g. (Graeme Thomas pers. comm. 2011). This leaves the roots
susceptible to attack from PRR at a critical time both physiologically and environmentally, when trees
are stressed following flowering and roots should be flushing and during the wettest time of the year,
when the Phytophthora pathogen is most active.
Most phosphonate research has been undertaken on ‘Hass’ in south-east Queensland and northern
New South Wales. Limited research on other cultivars has demonstrated that different application and
different timings are required to ensure translocation to the roots (Dann, 2011). No specific
phosphonate research trials have been conducted on Shepard.
Shepard avocados account for approximately 18% of the total Australian avocado crop with 45% of
Shepard avocados produced in the Atherton Tablelands and Mareeba/Dimbulah production areas.
Discussions from the ‘study groups’ workshops suggest that Shepard phenology is not fully
understood in north Queensland. It is expected that the results from this work will be applicable to
Shepard avocados grown elsewhere in Australia and reinforces the importance of monitoring root
phosphonate levels across all Australian avocado production areas.
To achieve the required phosphonate concentration in avocado roots for good PRR management
requires an understanding of tree phenology and monitoring and trial work to determine when a ‘top
up’ treatment of phosphonate can be applied to maintain the 25-40 µg/g concentration in the roots.
This study was conducted across five Shepard avocado orchards in far north Queensland, on sites
representing the range of local growing environments, from May 2012 to May 2013. In all five
orchards trees continued to receive the farmer’s standard phosphonate trunk injection program (either
1 or 2 injection
