By Graeme Thomas, GLT Horticultural Services

The basic physiology of an avocado tree in relation to where it sends energy over its lifetime is basically flawed.

If you look at the energy distribution of a young tree compared to a tree in full production, combined with the fact that the tree is very susceptible to Phytophthora root rot, there is little wonder that the average Australian production is less than 10 tonnes/hectare.

Energy distribution

Logic will tell you that if you do not have a healthy root system, your trees will not produce to their maximum. Fertiliser is not as effective, watering is not as effective. Production is consistently higher from trees with healthy root systems. Significantly higher and far more profitable.

When we have growers who pay very close attention to managing root health, as well as the other major areas of avocado agronomy, in fertilising to the crop needs and precise water management again to the needs of the tree, they can sustain yields of 30 tonnes/ha.

Research into the use of phosphorous acid for root rot control in the avocados started in 1977; by today’s standards, we were very naive. You only have to look at the label for Aliette (the first phosphonate fungicide registered) label to realise that our understanding of the physiology of an avocado and mode of action of phosphonates was limited, and we were very wrong in our approach back then.

Phosphonate needs to be applied when the tree is sending energy to the roots. The phosphonate moves to the roots with the energy flow. If it is applied when young fruit is developing, it will move to the fruit leaving a residue in the fruit. This residue will persist in the fruit until harvest. Dr Liz Dann, Queensland Alliance for Agriculture and Food Innovation (QAAFI) Senior Research Fellow, is investigating fruit residues in her new project Improving avocado orchard productivity through disease management (AV16007). Many growers across all growing regions have been contacted regarding sending fruit for analysis.

With knowledge of the phenological cycle in each growing region of Australia, we have been able to show when the optimum time is to apply a phosphorous acid product. It needs to be applied at a time that maximises the translocation to the roots. The fruit residue analyses will help to reiterate the message on correct timing of application.

With the benefit of monitoring root phosphorous acid levels, we have been able to set a level that will then persist in the roots, when the phenological cycle (the annual tree life cycle) prevents us from making further applications. With the good growing conditions of the Atherton Tablelands in Queensland, the root systems will continue to grow through the cooler months and dilute the level of phosphorous acid much quicker than other growing areas in Australia. This same situation occurs in cooler southern regions where we have vigorous non-bearing trees.

Application windows

There are two application windows. The most effective application window is when the summer leaf flush has hardened, and roots are actively growing. This window is the longest and most effective for phosphorous acid translocation. It ends when you are six weeks from flowering, so that phosphorous acid does not move to the developing flower and fruit.

The second application window is after the spring leaf flush has hardened up to when the summer flush commences. This period is short and less effective, and may result in some phosphorous acid translocating to developing fruit. Sometimes the summer flush will start immediately after the spring flush. Late applications will only result in phosphorous acid being deposited in the new summer flush. This application window should be regarded as one only to top up the root levels.

Phosphorous acid levels

We are very lucky that when phosphorous acid is in an organ, whether it is a fruit or a root system, it will persist there, until it is lost through natural root death, leaf fall or fruit harvest.

We have been able to show that when we lift the phosphorous acid level in a root system above 150mg/kg at the end of the summer flush application window, it will persist there at effective levels right through until the beginning of the next major application in the autumn/winter the following year. In all areas other than the Atherton Tablelands and young vigorous trees in Central Queensland, the top up at the end of the spring flush is usually not necessary.

We have also found that the number of treatments required to lift the root phosphorous acid levels back to150+ mg/kg is significantly less if you are starting from 70-80mg/kg as opposed to starting the application program from 5-10mg/kg.

If your levels at the end of your summer flush were 5-10mg/kg you would need 6-8 sprays and an injection to get you to 150mg/kg in one season. In contrast to that, if you were starting your application program with a root phosphorous level of 80mg/kg results have shown that three sprays will return the levels back to 150 + mg/kg.

Sampling

It is therefore not only cost effective but good management, to know what your root phosphorous acid levels are before commencing your application program. You may need three sprays or you may need eight sprays and an injection to achieve levels to produce a healthy root system.

When sampling roots, it is very important to take samples across the block being sampled. The more roots you take the less chance of sampling error and the more indicative the result will be to your block. For analysis, the laboratory requires 5g of roots; it is not a problem for the lab if you send in more. The other factor to ensure when sampling roots, is to only take white roots. It is the white root that absorbs nutrient, it is the white root where the phosphorous acid is deposited and it is the white roots that Phytophthora attacks. Including dark roots in the sample will lower the result.

Successful plantings

It has come to my attention recently that there are many growers planting trees with very poor success rates. This is coming from all growing areas from North Queensland at one extreme to the south-west of Western  Australia in the other and it is coming from both new and established growers.

With the cost of trees initially, plus the difficulty of establishing new trees amongst older trees or in replant land, tree deaths after planting are something that needs to be avoided. Trees have to be treated like newborn babies at planting time and for several months afterwards.

Simon Newett, from the Queensland Department of Agriculture and Fisheries, worked with industry to put together a YouTube video on the planting of avocado trees. It is well worth taking five minutes of your time and watching it.

I would have also included a treatment with metalaxyl granules immediately after planting and then repeat that treatment in six weeks, particularly if trees have come from a non-ANVAS nursery, and/or are being planted into ex-avocado blocks.

There are a few things that I have run across that both new and old growers are doing that are making the establishment of new trees next to impossible.

One is the practice of some growers who dip the young trees in various solutions before planting.

This can be dangerous in that if the trees have been on the ground, it is easy for soil to be on the bottom of the pot or planting bag. That soil can contain Phytophthora. If you introduce the disease into the dip, you can effectively spread that to all trees that follow in that dip.

Another concern is that some growers are trying to dip with beneficial microorganisms or “biologicals”. The companies that sell this type of material, from my experience, will not have done any replicated scientific trials in avocados to prove what they are claiming. In addition, there are some question marks as to how good their quality control is in the manufacture of these cultured microbes. There is always a risk that some microbes may be introduced that is detrimental to root growth and tree survival. So if this was the case, and you were to dip the new trees in a solution of detrimental microbes, you’re making tree establishment more difficult.

It is my strong advice to do it simple and not complicate things. Ensure that a tensiometer goes in the root ball of a good tree and water to that. Do not put a heap of fertiliser in the bottom of the hole.

Just keep it simple.

This article was produced for Guacamole, 2 March 2018.