Refer to the Manage Phytophthora Root Rot Poster. Phytophthora root rot requires an integrated approach, this is illustrated by the ‘Pegg Wheel’ on this poster.
Application of phosphorous acid –
Phosphorous acid has been extensively researched and proven to be an effective fungicide; however, the recommendations for its use should be strictly adhered to in order for it to be effective.
The whole aim of this treatment is to get sufficient phosphorous acid into the feeder roots in order to protect them from Phytophthora cinnamomi.
Considerable effort was made to provide the best and most up to date advice for the use of phosphorous acid in the video Protecting your avocado trees from Phytophthora root rot and the poster Manage Phytophthora Root Rot Poster. The poster has been updated twice since its original release.
There are two methods of application, injection and foliar spray, but the same timing applies to both.
Timing of phosphorous acid applications
- The timing of any phosphorous acid application, injection or spray, is absolutely critical to achieve the necessary levels of phosphorous acid in the roots for protection.
- The time to apply phosphorous acid is when root growth is taking place – this is when there is a net movement of sugars and other resources to the roots to fuel growth. Systemic chemicals such as phosphorous acid will flow there at this time too. When this is happening, the roots are said to be a strong “sink”.
- There are two windows of opportunity for application (indicated by the red arrows in the growth cycle below):
- Short window – late spring when the spring leaf flush has fully grown and matured.
- Long window – autumn/early winter once the summer leaf flush has fully grown and matured, but no less than 6 weeks before flowering.
- Applying at other times is ineffective and a waste of resources e.g. if applied during leaf flush or flowering, the phosphorous acid will end up in the leaves or flowers respectively, not the roots. Phytophthora cinnamomi does not infect leaves or flowers.
- Application during the late spring window will result in some of the phosphorous acid going to the fruit because they are actively growing at this time, but if guidelines are followed MRLs in the fruit will not be exceeded.
Application of phosphorous acid by injection
- Injection is suitable for both sick and healthy trees.
- Prepare a 20% solution of phosphorous acid by diluting 1 part of a 600 g/L product (e.g. Agri-Fos 600) with two parts of water. Use a product that has been buffered to pH neutral.
- Measure the diameter of the tree and multiply the diameter by 15 to work out the total volume to inject in the tree.
Example for a tree with a 4 metre diameter:
||4 x 15mL = 60mL of solution per tree
- Divide the total volume calculated per tree by 20 to determine how many separate injection sites to use per tree. In this example: 60mL ÷ 20 = 3 injection sites
- Inject 20mL of solution into each injection site.
- Distribute the injection sites evenly around the whole circumference of the trunk (roughly a handspan apart) in order to get protection for roots around the whole tree circumference (there is no lateral movement in avocado). Each injection will only protect the segment of the root system under it.
||Space needles evenly around the circumference of the trunk, about a handspan apart. Each needle will only protect a wedge-shaped section of roots underneath it. No lateral distribution of fungicide takes place.
- Do not use higher concentrations of phosphorous acid as a short cut to reduce the number of injection sites – since there is no lateral movement in avocado this would result in segments of the root system being unprotected.
- In older trees it is OK to inject large limbs instead of the trunk but select limbs that are distributed around the whole circumference of the tree.
Tips for injecting phosphorous acid
- Water trees the night before injecting to encourage good sap flow and start injecting as early as possible the next day. Uptake usually slows right down late morning (transpiration slows as the stomata close), when this happens stop injecting for the day and start again the following morning.
- Do not inject at the points where branches join the main trunk nor immediately above or below previous injection holes.
- Use a cordless drill and ensure that the drill bit is an appropriate size for the injection equipment being used. Drill injection holes about 25 – 50mm deep at a downward angle of about 45°. Carefully withdraw the drill so as not to enlarge the bore of the hole.
- Draw 20mL of the phosphorous acid into the equipment then carefully and firmly screw the syringe tip into the drilled hole without enlarging the hole. A tight fit is necessary to ensure fungicide does not leak out the sides.
- Do not apply excessive hydraulic pressure as this may separate the bark from the tree. With low-pressure injection systems, holes do not have to be plugged after injection. Where low pressure systems (such as veterinary syringes, Chemjet® or Aongatete Avo-Ject® syringes) are used, leave them in place till all the chemical has entered the tree, this should take about 15 to 30 minutes. If it takes longer it generally means that either the applicator has not been properly inserted or the tree is not transpiring sufficiently (stomata closed due to dry soil and/or high temperatures). Leave the injectors in place for longer and if necessary, check soil moisture is sufficient and try again early the next morning. Once all the phosphorous acid has been drawn into the tree remove the syringe by unscrewing it.
- After treatment, flush all injection equipment with clean water to remove any chemical residues to prevent damage to rubber and plastic components.
Application of phosphorous acid by foliar spray
- Foliar phosphorous acid sprays are only effective and recommended for healthy trees to maintain their protection. The compromised canopy of a sick trees is unable to absorb sufficient phosphorous acid so the tree must be injected instead.
- Use a rate of 8.3 mL of a 600 g/L product per litre of water to produce a 0.5% solution of phosphorous acid (some of the older labels list a rate that gives a solution that it is only 0.17 to 0.2% which is ineffective).
- It is important to ensure that the spray mixture in the tank has a pH close to neutral to avoid leaf burn. You need a pH testing kit (many growers use a swimming pool testing kit). To neutralise an acid mixture, it is safest to use potassium bicarbonate e.g. ‘Agri-K 415’ (not potassium or sodium hydroxide which are extremely caustic and dangerous to use). Add the potassium bicarbonate little by little, keep the spray tank agitator going and re-test frequently (it will get to a point when it changes quite quickly after a small addition of the neutralising product).
- It is important to use sufficient spray volume to get enough chemical into the tree. For an orchard block of mature trees, you need at least 2,500 L/ha. Do not use wetting agents as they can lead to leaf burn.
- Multiple sprays (about four) need to be applied to be equivalent to application by injection.
Tips for phosphorous acid sprays and why failures occur
- Multiple sprays are required to be equivalent to one injection, be guided by root analysis results. It is much harder to lift the root phosphonate from a very low level but once concentrations are at or close to the desired level they take far fewer spray applications to maintain.
- Use the correct rate: 8.3 mL of a 600 g/L product per litre of water to produce a 0.5% solution of phosphorous acid
- Apply a spray volume of at least 2,500 L/ha per spray on a mature orchard to get enough phosphorous acid into the trees.
- Correct timing is essential but is the same for both injection and spray methods.
- Do not add wetters or stickers.
- During hot weather, phosphorous acid sprays may react with copper fungicide residues (especially copper hydroxide products) on the leaves and cause leaf burn.
Testing phosphorous acid levels in the roots
- Regular testing of the white feeder roots is highly recommended to guide phosphorous acid applications.
◦ Spring – two weeks before spring leaf flush hardening (to determine if phosphorous acid needs to be applied in spring)
◦ Summer – two weeks before the start of autumn phosphorous applications (to guide applications)
◦ Late autumn/early winter – four weeks after autumn application (to see if target levels have been achieved or if more needs to be applied)
- Collect 5 grams of white feeder roots per sample. Access the two necessary forms by clicking here and answer all the questions, the more complete your answers the more reliable will be the recommendations from the interpretation. Send the root sample and the two completed forms to: MA Analytical Services P.O. Box 3104 Tarragindi Qld 4121
- Levels need to be maintained above 80 mg/kg for the whole season to protect the roots, however since levels decline naturally during the season (this is particularly rapid in the warmer, wetter regions such as in North Queensland) this means achieving much higher starting levels by the end of the main application period of autumn/winter (make sure all applications cease at least six weeks before flowering).
- At the end of the autumn/winter application window aim for a root level of >200 mg/kg.
- If levels are above 150 mg/kg prior to the spring application window then an application at this time is not required, unless your farm is in North Queensland in which case both spring and autumn applications should be applied (levels decline more rapidly in this environment).
- For further detail about the testing service please contact Graeme Thomas, GLT Horticultural Services: 0419 977 267 or email: email@example.com.
Use of metalaxyl
This fungicide acts directly on Phytophthora in the soil and will kill some but not all the Phytophthora inoculum in soil. It is also taken up by the roots, acts systemically to have curative properties. Trade names include Medley®, Ridomil Gold® and Zeemil®.
- Newly planted trees and older badly affected trees can be treated by spreading metalaxyl on the soil surface under the canopy and lightly watering it in. It is not likely to be cost effective for treating a large number of declining trees.
- Follow the label rate and instructions.
- Metalaxyl is effective for the first one or two applications but the response from subsequent applications is less beneficial because micro-organisms that degrade it build up in the soil.
- It is leached from the root zone by heavy rain or irrigation.