Injecting Trees with Phosphorous Acid for Dieback
by Peter Coppin, 2009. Revised 2011 & 2012.
Phosphorous acid is successful in treating a range of dieback diseases, in particular Phytophthora cinnamomi (Jarrah dieback) and Seiridium cardinale (Cypress canker or Conifer dieback). 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.
Timing of Treatment
Most of the serious dieback 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 diebacks 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, i.e. 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.
Treatment Dosage
Curative treatments should apply a minimum of 1ml of active ingredient per metre of canopy. If the tree is taller than wider, work on the height and vice 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 | 400g/l | 600g/l |
|---|---|---|---|
| Example products | Phos-inject 200, Yates AntiRot | Chemfos 400, Agri-fos 400 | Agri-fos 600 |
| Curative | 6 ml/m canopy | 3 ml/m canopy | 1.5 ml/m canopy |
| Preventative | 3 ml/m canopy | 1.5 ml/m canopy | 0.75 ml/m canopy |
| Mixing rate — Avocados & Conifers | 1:3 | 1:5 | 1:7 |
| Mixing rate — Native species | 1:4 | 1:8 | 1:12 |
For example, using a product with an active ingredient strength of 600g/l, curative treatment for a 10m tree would require the injection of 15ml of the product (10m x 1.5ml). At a mixing rate with water of 1:7, this is a total of 105ml of solution (7 x 15ml). 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 (e.g. Bordeaux, Copperoxychloride, Cupric hydroxide) 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.
Efficient and cost-effective Phytophthora control using phosphonate fungicides is based on understanding and working with the phenology or growth phases of the tree. 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.
Timing for Avocados
There are two distinct times when avocado trees will move phosphonates to roots. These being after the spring and summer shoot flushes. Experience supported by phosphonate root analysis has shown the best results are from applications after the summer flush. 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. Always distribute the injection sites evenly around the tree. The best practice is to dilute the current 600g/litre product back to 200g/litre phosphorous acid.
Root Phosphorous Acid Levels
Recommendations are based on maintaining a concentration in excess of 25 mg.kg-1 of phosphorous acid in roots to manage the disease.
Phytophthora cinnamomi will attack avocado roots any time soil temperatures are higher than 15°C providing there is sufficient moisture present to support growth. 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 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.
- Nutrition — strategic use of gypsum while avoiding application of large quantities of fertilisers containing ammonia.
- Mulching — 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.
See our Chemjet Tree Injector product page for the equipment needed to carry out trunk injection, and browse our other case studies for more real-world treatment examples.
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The Chemjet® Tree Injector and Potassium Phosphite work together to protect your trees using the methods described above.