Phytophthora root rot (PRR) poses a major challenge to the Australian avocado industry, costing over $20 million annually in lost production, disease control and tree replacement according to a recent survey by the avocado industry.

Decades of research have been invested in finding ways to overcome PRR, yet prominent avocado agronomist, Graeme Thomas, feels the extensive knowledge gained is not being fully adopted.

“Pioneering studies by Ken Pegg and Tony Whiley dramatically improved understanding of avocado tree physiology and PRR pathology in Australia.

“In his widely regarded ‘Pegg Wheel’, Ken proposed an integrated approach for PRR management involving appropriate site and rootstock selection, nutrition and irrigation management, and a need for ongoing chemical control.”

Graeme has been a strong supporter of these principles since entering the industry in the late 1970s, when he was involved in trial work to support registration of Aliette– Australia’s first phosphonate fungicide for combatting avocado PRR.

He remembers an industry in turmoil.

“At that time, Australian avocados were predominantly grown to the north and south of Brisbane.

“The 1974 Brisbane flood created perfect conditions for PRR and around half the trees in these regions died as a result.”

Phytophthora is more than just a fungus – it’s classified as an oomycete, which basically means it produces spores with flagella (or tails) that allow it to ‘swim’ towards plant roots. When soils become waterlogged from flooding, over-watering or poor drainage, the risk of PRR spreading to other trees is high.

After the 1974 flood event, it became clear the standard practice of stumping or removing trees when they started to decline was not economical – three years of production was lost waiting for replacement trees to establish.

What followed was a determined effort to find effective treatments for protecting trees from PRR.

The advent of phosphonate fungicides in the 1980s began with registration of Aliette as a foliar spray for avocados in 1982. It was soon followed by the release of the less expensive potassium phosphonate in 1985, which is still widely used today as both a foliar spray and trunk injection.

“Phosphonates have a dual mode of action. In the tree, they are converted to phosphorous acid which is directly toxic to phytophthora and also triggers an immune response in the roots,” explains Graeme.

“Being mobile in both the xylem and phloem vessels makes it one of the few truly systemic fungicides.

“This mobility allows growers to either inject the fungicide into the trunk, where it moves to the leaves before being transferred to the roots, or apply it as a foliar spray, where it is absorbed through the stomata before moving to the roots.

“If applied to the soil, it will slowly oxidize to phosphoric acid which then has no activity as a fungicide against PRR.”

The key to effective foliar application is to ensure contact with the underside of the leaves, as this is where the stomata are located. Equally important is applying the fungicide when stomata are open to facilitate uptake into the leaf.

Then there is the matter of getting it from the leaves to the roots.

“Application needs to be done when the tree is sending energy to the root system,” says Graeme.

“At different growth stages of the tree, such as in a vegetative flush or at flowering time, there is a greater proportion of phosphorous acid going to the parts of the plant that are growing quickly, and not the roots.”

These were hard lessons learnt in the early years of phosphonate fungicide use.

Results were often variable which prompted Graeme to ask SGS in Toowoomba to develop a reliable test for measuring phosphorous acid concentration in avocado roots.

Since its introduction in the late 1990s, this test has shown that effective protection against PRR is achieved when root phosphorous acid levels are 80 mg/kg or above.

Another flood event, this time in Bundaberg, caused a significant loss of avocado trees in 2010/11.

“That loss of production, plus increased demand for avocados, saw supply not meeting demand until around 2019,” says Graeme.

It served as reminder that PRR can have dire consequences if not properly managed.

And that’s exactly what concerns Graeme.

“From the national root analysis results, it’s clearly seen that orchards in Eastern Australia are struggling to meet the 80 mg/kg target for root phosphorous acid levels”.

The current situation means these orchards are vulnerable to major production losses when the next flood event occurs.

National root analysis results show orchards in Eastern Australia are struggling to meet the 80 mg/kg target for root phosphorous acid levels.

Graeme believes the difficulty in attaining adequate root phosphorous acid levels in Eastern Australia stems from the higher soil temperatures encountered there.

“Avocado root growth occurs between 21 and 33°C. Low soil temperatures in cooler regions such as south-west WA restrict root growth from April to November, making it a simple process to maintain root phosphorous acid levels.

“However, in regions such as North Queensland where soil temperatures are high for most of the year, root phosphorous acid levels will rapidly decline due to uninhibited growth dilution.”

In 2017, Graeme began investigating ammonium phosphonate as a potential treatment for combatting this issue.

He wanted to know whether its successful use in South Africa could be replicated in Australia.

By 2023, trials of UPL Australia’s Prowess ammonium phosphonate fungicide had been conducted across four locations in Queensland and WA, all confirming that foliar applications of ammonium phosphonate outperformed conventional potassium phosphonate foliar treatments in terms of increasing root phosphorous acid levels.

In 2024, Prowess became the only 100 per cent ammonium phosphonate fungicide registered in Australia for use as a foliar spray against PRR in avocado.

Like all phosphonate products, its efficacy depends on application at the right time and under the right conditions.

“The advantage of using Prowess is the better phosphonate uptake it provides, which makes it easier for growers to maintain adequate root phosphorous acid levels,” explains Graeme.

“If these levels are not maintained and tree health starts to decline, it can take years to get it back.”

With extreme weather events like floods becoming more frequent, Graeme is adamant that we learn from the past and prioritise root health to safeguard against future production losses.

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Wednesday, 23rd August 2028

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