Possible cure for kiwifruit disease

Last updated 12:35 17/04/2012
Dave Leonard and Tom Harris
PATRICK HAMILTON
RICHMOND BUSINESS PARTNERS: Dave Leonard and Tom Harris, of Biological Solutions Ltd, in a Hope kiwifruit orchard.

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A biological product made by a small Richmond company is showing promise in trials to find a compound that can control the rampant kiwifruit vine disease Pseudomonas syringae pv actinidiae V (Psa-V).

Hundreds of chemical and biological agents have been tested by industry-government biosecurity agency Kiwifruit Vine Health (KVH) and independent laboratories in the last year to counter the bacterial disease which has spread to more than 40 per cent of kiwifruit orchard land, almost all of it in the Bay of Plenty.

It is expected to cost the industry about $120 million in lost export earnings this year.

The high-value, high-cropping gold variety, hort16a, in particular, has been decimated by Psa-V, forcing Zespri to hurriedly replace it with other new gold varieties which growers have begun to graft on and plant.

Biological Solutions, a company run by Dave Leonard and Tom Harris, which specialises in supplying a range of beneficial bacteria and fungi products for horticultural use, came up with Plant Shield more than a year ago after being asked by some Bay of Plenty kiwifruit clients to find something that would help fight Psa-V.

A combination spray of three strains of naturally occurring, soil-based bacteria – Bacillus subtilis, Bacillus pumilis and Pseudomonas fluorescens – commonly used to treat fruit diseases such as botrytis, downy and powdery mildew and rot, it has been shown to be effective in controlling Psa-V in laboratory tests.

Jenny Aitken, a plant physiologist with more than 30 years' experience, who now owns The Tree Lab in Rotorua, said Plant Shield was one of the three most successful of about 60 chemical and biological compounds she had tested.

While it was early days, she was "really excited by it".

She had now sent samples to Crown research institute Plant and Food to conduct greenhouse trials, while growers were testing it in the field.

Dr Aitken, who outlined her research at the recent international Biotech Fruits 2012 symposium in Nelson, said the aim now was to refine Plant Shield further, investigate the best way to apply it and make it more cost effective for growers to use.

As a natural solution, it appealed not only to organic growers, but to those who had already used a lot of chemicals on their orchards in a bid to counter Psa-V, she said.

"They have been spraying a lot more than they normally would, so the plant's natural defence mechanism falls, just like we do if we use antibiotics too much."

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Mr Leonard, a former representative cricketer who also owns a company producing seaweed fertiliser, said they had spent tens of thousands of dollars testing Plant Shield, including by KVH.

However, KVH was having to repeat its trials of many products because it had used too low a dose of Psa-V, so while it waited for them to be redone, Biological Solutions had organised parallel and more detailed tests with Dr Aitken's laboratory, which followed the same protocols, he said.

Mr Harris, who has been involved with fish fertilisers and sprays for many years and has a joint venture with Sealord, said many growers thought KVH was moving too slowly and they were willing to pay for good information from other laboratories.

Tests showed their product not only protected the plant by boosting the number of beneficial bacteria colonising it, but attacked Psa-V, which he was confident plant trials would confirm.

KVH seemed focused on finding a kill cure rather than also taking preventive steps which boost plant and soil health, he said. Growers needed to take a more empathetic, holistic approach to their orchard environment.

"We are not going to win by fighting a rearguard action against this disease, as it's always going to be stronger and bigger because it's airborne, soil borne and water borne, so we have to find another way to activate the plant's own defence systems."

In the meantime, KVH was heavily promoting the use of copper sprays, but they were indiscriminate in their effect, killing good along with bad bacteria, Mr Harris said.

Overuse or long-term use could also lead to stunted leaf growth and small fruit, as well as leaving a buildup of the heavy metal in the soil and in runoff.

Mr Leonard said the use of chemicals to control diseases was becoming increasingly problematic as recommended withholding periods grew and overseas markets demanded fruit with little or no residues and used them as "pseudo trade barriers".

He questioned how much longer the kiwifruit industry could use streptomycin, an antibiotic banned in Europe, as a pre-flowering spray for Psa-V.

While biological agents had their limitations in the middle of winter when there were no leaves on vines and the plants were dormant, they could be highly effective used in conjunction with other solutions, including copper, he said.

The wine industry had shown it was possible to move en masse to organic production and still cope with major diseases such as botrytis, Mr Leonard said.

"We expect Psa-V to remain an issue no matter what the cultivar and that people will have to use some sort of programme to provide protection."

That included in Nelson, which was currently free of Psa-V, as the healthier vines were, the better able they were to cope with the disease when it arrived, he said.

CHINA LIKELY CULPRIT

Scientists are on the brink of unravelling the origins of Pseudomonas syringae pv actinidiae V (Psa-V), the disease devastating New Zealand's kiwifruit vines, and China is the most likely culprit.

A team of the University of Otago biochemists is confident it will be able to confirm the source of the virulent disease, which was first detected here in November 2010.

The scientists were able to map out the bacterium's genetic code using a multimillion-dollar advanced genetic sequencer, provided through the Government-funded New Zealand Genomics Ltd (NZGL).

The results have ruled out one of the main suspect, Italy, which also puts to rest any accusations that New Zealand was the source of historic Italian outbreaks. Samples of the Chinese Psa strain have also been sequenced, with the results indicating China is the source of both the New Zealand and Italian outbreaks.

The team is currently testing samples from the third main suspect, Chile, with preliminary data showing that it, too, originated from the Chinese strain. The disease was unlikely to have entered New Zealand on footwear or through quarantine channels, so how it got here needed to be established, said University of Otago Department of Biochemistry associate professor Russell Poulter.

"We need to know the pathway, so we can help other countries and ourselves avoid similar mistakes in future," he said. Mr Poulter said the Psa outbreak highlighted how critical NZGL technology was for New Zealand, with an economy based largely on agriculture and horticulture.

"If NZGL wasn't here, we would never have been able to sequence the bacterium and answer the industry's questions."

NZGL was set up two years ago as collaboration between Otago, Massey and Auckland universities, and has $40 million in government funding committed over 10 years.

As the researchers continue to investigate the origins of the disease, they will also research the optimal treatment of Psa with antibiotic sprays.

- Nelson

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