Wednesday, July 6, 2022

Grower challenges copper spray value

A Bay of Plenty kiwifruit grower claims advice on Psa control threatens any viability remaining for the high value Gold fruit.

Russell Baker has a small orchard in the Psa “ground zero” zone of Te Matai road zone, the first area to be affected by the disease.

Long an ardent critic of recommendations around copper spraying as a means of slowing the disease, he is claiming his small remaining unsprayed vines are testimony to copper’s ineffectiveness.

Baker’s concerns over the cost and effect of copper treatment have also been echoed by other growers and leading Psa researcher, Dr Russell Poulter, an Otago University associate professor in genetics.

Poulter cracked the Psa genome, isolating it to a strain linked to Chinese sourced bacteria.

Baker said his vines had not had any significant copper spray application since last winter “and they look like one of the best blocks around”.

Multiple copper treatments have been recommended by Kiwifruit Vine Health (KVH) as a means of managing Psa control. However, this best practice regime has been challenged by Baker on grounds it has not been proven in orchard split block trials.

“The glasshouse trials used to date are simply too artificial to represent what happens in an orchard situation and until proven in the orchard there is no such thing as a ‘proven’ programme,” Baker said.

However, Zespri and KVH chairman Peter McBride said split block trials were problematic when trying to replicate the impact of Psa in a controlled environment.

“Controlling the level of Psa inoculum is difficult in an orchard trial, you do not get an even controlled level across the block.”

Baker is currently in a dispute with Zespri over whether his tiny remaining .2ha block of Gold fruit constitutes a commercial crop requiring payment of a G3 licence, amounting to around $50,000.

He maintains he wants to keep the crop in to monitor its response over the sensitive spring period, known to be a challenging time for vines facing Psa infection.

A year ago Poulter said the Bay of Plenty was getting a “blue tinge” from the huge amounts of copper spray being applied to kiwifruit crops. He cautioned the disease was still spreading and Psa appeared resistant to copper treatment.

A year on, over 60% of the country’s orchards have Psa despite the copper spray programme and he believes his concerns have been proven. Poulter is urging the industry to look harder at antibiotic based Streptomycin spray again.

The spray was used last year, but created controversy when some growers misapplied it, resulting in a residue scare. Its control has since been tightened for use only before flowering.

“Copper does offer some protection. But will copper on its own do it? I very much doubt it. I would say Streptomycin is necessary to produce a commercial crop. If it was a choice based on no crop, or a crop with no Strepto’ residue, I know what I would do.”

He is concerned growers have thrown millions into the new G3 variety, but without robust and proven spray programmes in place, “you are bleeding the sector out”.

But McBride maintains those growers who have adhered to a strict regime of copper application and good orchard practices are still remaining on top of their Psa infection.

“I know of three growers in Opotiki who are able to continue because of their effective copper spraying programme.”

Streptomycin is registered for use on a dozen fruit and vegetable types for the treatment of blights and bacteria, including apples and pears. Greater use on kiwifruit would require approval from the Ministry for Primary Industries (MPI).

Te Puke grower Fred Harvey spent time in Italy recently where 70% of orchards are affected by Psa. Copper sprays were used there, but are now only applied over winter.

“The comment was from the Italians they had visits from other affected countries, but not from New Zealand to see how they were coping with the disease.”

Italian growers pointed him to Princeton University research by Bonnie Bassler showing the ability of bacteria to adapt to challenges, and this was how Psa was able to withstand the assault by copper sprays.

By late November Psa was affecting 1995 orchards and 68% of NZ’s kiwifruit hectares were on an orchard identified with Psa (see accompanying map).

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