You are here: home
Preventing Dieback

Injecting Trees With Phosphorous Acid for ‘Die-back

by Peter Coppin, 2009. Revised 2011 & 2012.
Phosphorous acid is successful in treating a range of ‘die-back’ diseases, in particular Phytophthora cinnamomi (Jarrah die-back) and Seiridium cardinale (Cypress canker or Conifer die-back). It is available in a range of products, where the active ingredient is present as mono & di potassium phosphonate/phosphite, which in its raw form is a safe, non-scheduled compound. Trees also get a slight nutritional boost from treatment, which also helps combat diseases.
Most of the serious die-back diseases sporulate and spread in spring and early summer, and do the most physiological damage to plants during those times. Foliage symptoms often do not appear until mid summer through to late autumn. This is because die-backs cause restrictions/impediments to moisture uptake and translocation by killing/choking the vascular vessels, and it is in the summer months that the plants are under the greatest moisture and heat stresses.
For this reason, the main treatments should be applied in spring and early summer and repeated eight to ten weeks later. Treatments are always more effective when applied during periods of active sap flow, ie, when plants are in a vegetative growth stage (often called the spring flush). During warm to hot weather, treatments should be applied before 10am.
Evergreen trees also have an autumn flush of growth, usually from late March to early May, so a follow-up treatment at this time can be beneficial. In fact, this is often the first treatment because symptoms of disease may not have been noticed until mid to late summer.
Curative treatments should apply a minimum of 1ml of active ingredient per meter canopy. If the tree is taller than wider, work on the height and visa versa. Treat the infected tree and its immediate neighbours at full strength. Half rates can be used on other trees as a preventative measure. For trunk injection, the table below shows application rates according to the active ingredient concentrations of varying products.
Product strength:
200g/l eg,
400g/l eg,
600g/l eg,
Phos-inject 200®
Chemfos 400®
Agri-fos 600®
Yates AntiRot
Agri-fos 400
6mls/m canopy
3mls/m canopy
1.5mls/m canopy
3mls/m canopy
1.5mls/m canopy
0.75mls/m canopy
Mixing rate with water:
Avocados & Conifers:
Native species:
For example, using a product with an active ingredient strength of 600g/l, curative treatment for a 10m tree would require the injection of 15mls of the product (10m x 1.5mls).
At a mixing rate with water of 1:7, this is a total of 105mls of solution (7 x 15mls).
Using 20ml syringes, this means five injections evenly spaced around the trunk. Injections should ideally be before any branching, but branches coming from very close to the base of the tree can be treated separately.
NB: Alkaline water can neutralise acid based compounds, so use distilled water or rainwater.
20ml syringes take from 20 minutes to two hours to empty, depending on what percentage of the canopy has died and on the level of sap -flow. Trees do not have to have all the treatment at once, reducing the number of syringes needed, but ideally a single specimen would have all its injections within 24-48 hours.
With Cypress canker, copper-based fungicides (eg, Bordeaux, Copperoxychloride, Cupric hydroxide, etc) have proved to be very effective as preventative sprays in mid to late spring, especially on smaller trees where a thorough coverage can be achieved. Always add a wetting agent to assist with effective coverage and spread of the spray. Aim for branch unions more than just a leaf/foliar spray.

Spraying a tree is difficult and expensive and restricted by the weather conditions, equally 'watering in' if a feasible treatment is less effective, expensive and environmentally damaging .

Using Phosphonates Effectively to Control Phytophthora Root Rot in Avocados

by Graeme Thomas.


It was discovered that a simple chemical called phosphorous acid (H3PO3) was more effective in restoring health of diseased trees when measured amounts were injected directly into tree trunks. Due to the acidic nature of phosphorous acid significant damage occurred around injection sites, so a safer formulation of potassium phosphonate was developed and is now commercially available from several companies who market the product in Australia. The knowledge that has accumulated since this time, both on tree physiology and its relationship with movement of the fungicide within the tree is significant. However, it is disappointing that many avocado growers do not correctly apply current technology in the management of root rot today.


Efficient and cost-effective Phytophthora control using phosphonate fungicides is based on understanding and working with the phenology or growth phases of the tree (Fig. 1). When potassium phosphonate is applied to trees it enters the tissues and becomes completely systemic moving within the vascular system to all parts of the tree. However, the quantity found in various organs will depend on the stage of growth at the time of application. This is due to different parts of the tree having stronger or weaker access to resources. For example, small, fast-growing fruit has the highest priority for available nutrients, shoots and leaves the second highest while roots have the lowest priority. If young fruit are developing on the tree at the time of treatment (during October early November in Fig. 1) most phosphonate applied will move

to these fruit with very little reaching the roots. Indeed research has shown that fruit may have phosphonate concentrations in excess of 280 while roots only finish with 7-10 Since we are controlling a root disease, the fungicide needs to be applied when the tree is diverting most of its resources to roots.


As can be seen, there are two distinct times when avocado trees will move phosphonates to roots. These being after the spring and summer shoot flushes. If phosphonates are applied outside these times, a significant percentage will be translocated to the non-target organs. When making an application after the spring flush, there will be reduced competition from leaves. However, in many regions the summer flush follows on very quickly and will compete more strongly for the phosphonate so the amount making it through to the roots may be quite low. Experience supported by phosphonate root analysis has shown the best results are from applications after the summer flush, when competition from other organs is basically non-existent. Recent research by QDPI has shown that elevating the phosphonate levels in the roots prior to a new root flush can reduce final root volume. It is therefore advised to wait for root development to complete prior to applying phosphonates. In most regions and with most varieties, this is late autumn to early winter (May to June).


It should also be remembered that even though phosphorous acid is systemic, moving from roots and leaves to other organs, it does NOT move laterally around the tree. This should be remembered when injecting. Many growers are now using concentrated product to reduce the number of injection points. This is truly a false economy as you are overdosing a small area of roots and leaving large areas of roots untreated. Always distribute the injection sites evenly around the tree. To do this effectively, the best practise is to dilute the current 600g. / litre product back to 200g. / litre phosphorous acid. This situation has been clearly demonstrated with phosphorous acid levels in the roots under a single tree varying from 5mg./ kg. to over 200mg./kg.


When phosphonates were first registered as a foliar spray application results were inconsistent. As research continued, it was found that spray volumes were critical. Low volume applications were not lifting root phosphonate levels to the same degree as high volume applications. It has since been found that it is essential to get thorough coverage not only to the leaves, but to branches and trunk as well. In mature orchards the volume needs to be as high as 3000 litres/ha. Younger orchards with smaller trees will require less.


Root Phosphorous Acid Levels


Prior to the development of the test measuring avocado root phosphorous acid concentrations (SGS, Toowoomba, QLD), recommended applications were based on tree response to treatment. There were many unknown variables that made results less predictable. Integrating root phosphonate analysis with application strategies increased confidence in protecting trees and controlling Phytophthora root rot in the orchard. Recommendations are based on maintaining a concentration in excess of 25 of phosphorous acid in roots to manage the disease.


Talking Avocados


Phytophthora cinnamomi will attack avocado roots any time soil temperatures are higher than 15 C providing there is sufficient moisture present to support growth. Hence, there will be times during the year when root protection is required but applications of phosphorous acid will not reach roots in sufficient concentration due to the competition from growth by other plant organs. This includes the critical flowering time when significant stress is imposed on trees and healthy roots are required. To achieve continuity of root protection for the longer term an elevated concentration of root phosphorous acid is required. Monitoring root concentrations following phosphonate applications has demonstrated that this is best built during the late autumn and early winter months.




Other Management Factors


Fungicide management is only one factor in the control of Phytophthora root rot. The integrated management technique as best described in the “Principles of Phytophthora root rot management” must also be incorporated in your management program.


In mature trees, the key factors are:


    Irrigation management – water consumption of diseased trees is dramatically reduced compared with healthy ones in the same block. Where diseased trees are present in an otherwise healthy block reduce water application by installing lower volume sprinkler heads.


   Nutrition –strategic use of gypsum while avoiding application of large quantities of fertilisers containing ammonia.


   Use good mulches, which allow “breathing” while maintaining uniform soil moisture within the root zone. Waterlogging of soils will accelerate the development of the disease.