Managing avocado orchards affected by wet weather
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Avocado trees are sensitive to inundation, and if flooded for about 48 hours will die from lack of oxygen to the roots. These saturated areas are not necessarily confined to just the flatter parts of the orchard, spots where water flows and where springs emerge will also be adversely affected.
When safe to do so, inspect the orchard and mark areas that are affected by poor drainage. Take steps immediately to improve the drainage of these areas so that the water can get away.
In the longer term decide whether you can significantly improve drainage for the affected trees (eg mounding, improved surface drainage, installation of subsurface drainage). If not then remove avocado trees from the area, and use it for some other purpose. Building better mounds for future tree rows may help in marginal areas as long as they are orientated to allow the water to get away and not dam it up.
Phytophthora root rot
Avocados are very susceptible to Phytophthora root rot which thrives in saturated, cool soils. Conditions going into autumn after significant rain present the pathogen with ideal conditions.
After several days of cool, cloudy weather the ability of avocado trees to resist the disease is even lower because they haven’t been able to photosynthesize adequately to build up reserves of carbohydrate to fight against the disease and grow new feeder roots.
Phosphorous acid application
The timing of phosphorous acid applications is critical because it will go to the parts of the tree that are growing most actively at the time of application.
Phosphorous acid is needed in the feeder roots to protect them from attack so it must be applied when the feeder roots are actively growing, this generally happens once the most recent leaf flush is fully expanded and hardened. Late autumn till the start of winter (from about March onwards depending on your area) is the best time and longest application window of the year because this is when root growth is strongest.
Phosphorous acid can be applied by injection or multiple foliar sprays but only use the foliar method if the trees have a full and healthy canopy of leaves (this is needed to allow sufficient uptake of the chemical). In other cases, use the trunk injection method, the only exception being if the trees are too young to inject in which case use enough foliar spray volume to thoroughly wet the bark as well as the leaf canopy.
Using the correct rates, volumes and timing are all critical for phosphorous acid applications to be effective. Refer to the ‘Manage Phytophthora root rot’ poster for these details, available from firstname.lastname@example.org or Avocados Australia. It can also be viewed on the avocado Best Practice Resource (type ‘poster’ into the search box).
To see whether your phosphorous acid application practices have been effective, regularly test the phosphorous acid content of your feeder roots. If you don’t have root levels of 150ppm or more four weeks after your autumn applications or over 90ppm going into summer, then you need to re-examine your application practices against those described in the poster and re-apply.
Metalaxyl (eg Ridomil®) application
Metalaxyl directly kills Phytophthora in the soil once its spores start germinating and invading roots.
The timing of applications isn’t as critical as it is for phosphorous acid so it can be applied under the tree canopy anytime, however, growers should be aware that metalaxyl is easily leached beyond the root zone by heavy rain.
This chemical is expensive so do your sums on the cost/benefit before committing yourself.
Consider treating the worst affected areas. Also bear in mind that each subsequent application is less effective since micro-organisms that degrade it build up in the soil.
Anthracnose on the fruit
After a lengthy period of rainy weather sprays against anthracnose disease are likely to be behind schedule.
Registered strobilurin sprays (eg Amistar®) have some reach-back effect and can be applied three times per season (the restriction in the number of applications is part of the anti-resistance protocol) so it would be a good strategy to get a spray of this fungicide on as soon as you are able to.
Three of the important nutrients for avocados are prone to leaching from the soil and as a result of the high rainfall are likely to be low, these are:
Growers need to adjust their fertiliser applications to make up for expected shortfalls, typically rates are raised by up to 20% above normal, but be very careful with boron (especially on light sandy soils) not to overdo the rates since this element can easily reach toxic levels. Consult your agronomist for customised rates.
Fertiliser rates are best split into frequent applications of small amounts, this means that the amount that can potentially be lost with each rainfall event will be lower and the levels will be topped up sooner with the next application.
These sections of the BPR contain more information, and we’ve also provided links in the text above to other resources:
This article was prepared for the 1 April 2021 edition of the Guacamole.
Date Published: 26/03/2021