With vine disease Psa on the rampage in New Zealand, some scientists and growers are questioning the streptomycin spray programs endorsed by Zespri and Kiwifruit Vine Health (KVH).

Psa spring symptoms

Last week KVH reported new cases of Psa-V positive orchards in the areas of Katikati and Whakatane, while before that there were already 295 infected orchards in New Zealand.

While the majority of these are in Te Puke in the heart of New Zealand’s Kiwifruit industry, Psa-V has also been found in Tauranga and Waihi.

Scientist John Young, who spent his 40-year career in roles in the Plant Diseases Division at the Department of Science and Research and later at Landcare Research, says the disease can be present in blocks where there are no symptoms at all.

"The pathogen moves well ahead of physical symptoms. The visible disease is just the tip of the iceberg," he says.

New Zealand Kiwifruit Growers Incorporated (NZKGI) president Peter Ombler, says growers face a "very serious situation".

"It gets worse before it gets better, there’s no point in trying to hide from that fact. It means we have to fight, and we’re fighting it as fast as we can," he says.

In the battle against the disease, KVH has introduced a spray program which it subsidizes for growers to treat Psa-V.

Psa-induced chainsaw gang destruction

Treatment questioned

Some scientists and growers have alleged the recommended Zespri streptomycin treatment for Psa-V (virulent strain of pseudomonas syringae) is wrong and causing more damage than good.

Young, whose expertise is mainly in the epidemiology and control of pathovars of pseudomonas syringae and the disease's significance to crops, says he thinks the current regime is not supported by proper research.

The now retired scientist claims while KVH has about 20 experts working on solutions, there are none with expertise in bacterial pathogens of kiwifruit.

"We need evidence by scientists reporting field trials that they have proved are effective. But so far recommendations are being made without these trials," he says.

Zespri general manager of Psa Innovation David Tanner, told www.freshfruitportal.com as Psa has not been in New Zealand for a full year there has been no opportunity to access products on fruit production or vine growth.

"The trial work, on plants in greenhouses, shows efficacy of the products. These trials have their limitations and these limitations are well known to growers," he says.

Scientists queried

Psa cane death

Horticultural and environmental scientist Nathan Balasingham, believes Zespri has been misled by its own scientists.

Balasingham advocates a range of choices when it comes to Psa solutions, and has produced his own biochemical spray as an alternative to copper and streptomycin-based products.

"The worrying factor is scientists not speaking out more. There are hoards of scientific information that says don’t use copper during the growing seasons, don’t use streptomycin or copper against Pseudomonas syringae," he says.

However, Tanner says all scientists Zespri works with are professional experts in their field.

"In addition, Zespri/KVH have engaged a Scientific Oversight Committee which includes leading plant experts from around New Zealand and the world – these people give us the benefit of their many years of scientific experience, as well as access to networks that we would otherwise find it difficult to engage."

Nowhere to go

Young says it is likely that growers, who understandably do not fully understand the science, will do whatever is recommended by Zespri.

"I mean kiwifruit growers are desperate and understandably so, and they will do anything that is a recommendation. Ill-informed recommendations are coming from almost anywhere."

Tanner says a spring Psa management program has been released and it is for growers to then choose how best to use the information.

"The fact that only a small number of growers have decided to use KeyStrepto demonstrates that orchardists are making their own decisions on how to best protect their orchards."

Russell Baker, a grower from the Te Puke region says he will not be using copper on his vines, saying he has been frustrated at what he sees as 'waffly' answers from Zespri.

"It’s frustrating because they want to back track and there’s always some excuse why some test may have appeared to give that impression but no-one wants to give scientific evidence," he says.

Psa leaf symptoms

But Peter Ombler from NZKGI says he is disturbed by some accusations that growers do not have a choice in spray programs.

"I can’t stress enough that nobody is being forced do anything. I don’t know where that information is coming from."

Ombler says his organiation represents all 2,700 kiwifruit growers in New Zealand despite a breakaway group Independent Kiwifruit Growers Association (IKGA) forming in 2010, alleging a too cosy relationship between NZKGI and Zespri.

Tanner says Zespri is confident NZKGI has the support of the majority of growers.

“NZKGI are the mandated representatives of growers, they recently have subjected themselves to an industry referendum and received very strong support for their role – no other grower group subjects themselves to such scrutiny or transparency,” says Ombler.

Related story: NZ to launch new spray option for Psa-hit kiwifruit vines

Photos: Nathan Balasingham

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